There are two versions of this game. To find out what version your game is, look at the back of the cartridge. These are codes for the ASMEN0J20 version.
Game ID: ASME-f486f859

--------------------
Source: Codejunkies
Infinite Lives
02098930 00000063

Infinite Health
12090a00 00000880
121082a4 00000008

Infinite Coins
02098998 00000064

Press L+R+SELECT For 7 Red Coins
94000130 000000fb
1209894c 00000007
d0000000 00000000
9209894c 00000008
12098b4c 0000ff80
d0000000 00000000

Press B To Moon Jump
92096b5c fffd0002
0217f760 00019b23
d0000000 00000000
92096b5c fffd0002
0216e380 00019b23
d0000000 00000000
Do not use in cut-scenes or when going through doors.

Press Y To Slow Falls
92096b5c f7ff0800
0217f760 ffffc000
d0000000 00000000
92096b5c f7ff0800
0216e380 ffffc000
d0000000 00000000
Do not use in cut-scenes or when going through doors.

All 120 Course Stars
020960f4 ffffffff
020960f8 ffffffff
020960fc ffffffff
12096100 0000ffff
22096102 000000ff

100 Coins Total for Every Course
12096112 00006464
02096114 64646464
02096118 64646464
0209611c 64646464
22096120 00000064

All Castle Doors Unlocked
120960e4 0000ffff
220960e6 000000ff

All Mini Games in Rec Room
02096128 ffffffff

Mario, Luigi & Wario Never Lose Cap
220960e7 00000080

Test Map 1 (Hold Select While Loading a Save)
94000130 fffb0000
020988a4 00000029
d2000000 00000000
Code by Parasyte.

Test Map 2 (Hold Select While Loading a Save)
94000130 fffb0000
020988a4 00000000
d2000000 00000000
Code by Parasyte.

CHARACTER MODIFIER (Walk Through a Door):

Hold L+R+UP to Play as Yoshi
94000130 000000bf
22096121 00000003
d0000000 00000000

Hold L+R+LEFT to Play as Wario
94000130 000000df
22096121 00000002
d0000000 00000000

Hold L+R+RIGHT to Play as Luigi
94000130 000000ef
22096121 00000001
d0000000 00000000

Hold L+R+DOWN to Play as Mario
94000130 0000007f
22096121 00000000
d0000000 00000000

FLOWER POWER CODES:

Infinite Fire Flower Power Up Time
62094a90 00000000
b2094a90 00000000
100006be 0000ffff
d2000000 00000000

Infinite Feather Power Up Time
62094aa8 00000000
b2094aa8 00000000
1000065e 0000ffff
d2000000 00000000

Yoshi: Press L for Flower Power (Fire)
920966da 00000000
94000130 000001ff
92096120 00000300
62094a90 00000000
b2094a90 00000000
100006be 000003ff
d2000000 00000000
920966da 00000000
94000130 000001ff
92096120 00000300
62094a90 00000000
b2094a90 00000000
200006f4 00000001
d2000000 00000000
920966da 00000000
94000130 000001ff
92096120 00000300
62094a90 00000000
b2094a90 00000000
20000713 00000001
d2000000 00000000
920966da 00000000
94000130 000001ff
92096120 00000300
62094a90 00000000
b2094a90 00000000
20000714 00000001
d2000000 00000000

Wario: Press L for Flower Power (Metal)
920966da 00000000
94000130 000001ff
92096120 00000200
62094aa8 00000000
b2094aa8 00000000
1000065e 000003ff
d2000000 00000000
920966da 00000000
94000130 000001ff
92096120 00000200
62094aa8 00000000
b2094aa8 00000000
200006a9 00000001
d2000000 00000000
920966da 00000000
94000130 000001ff
92096120 00000200
62094aa8 00000000
b2094aa8 00000000
200006aa 00000001
d2000000 00000000

Luigi: Press L for Flower Power (Invisibility)
920966da 00000000
94000130 000001ff
92096120 00000100
62094aa8 00000000
b2094aa8 00000000
1000065e 000003ff
d2000000 00000000
920966da 00000000
94000130 000001ff
92096120 00000100
62094aa8 00000000
b2094aa8 00000000
200006ab 00000001
d2000000 00000000
920966da 00000000
94000130 000001ff
92096120 00000100
62094aa8 00000000
b2094aa8 00000000
200006ac 00000001
d2000000 00000000

Mario: Press L for Feather Power (Wings)
920966da 00000000
94000130 000001ff
92096120 00000000
62094aa8 00000000
b2094aa8 00000000
1000065e 000003ff
d2000000 00000000
920966da 00000000
94000130 000001ff
92096120 00000000
62094aa8 00000000
b2094aa8 00000000
200006af 00000001
d2000000 00000000
920966da 00000000
94000130 000001ff
92096120 00000000
62094aa8 00000000
b2094aa8 00000000
200006b0 00000001
d2000000 00000000

Any Character: Press START+R for Mario Power Up (Wings)
920966da 00000000
94000130 000002f7
62094aa8 00000000
b2094aa8 00000000
1000065e 000003ff
d2000000 00000000
920966da 00000000
94000130 000002f7
62094aa8 00000000
b2094aa8 00000000
200006af 00000001
d2000000 00000000
920966da 00000000
94000130 000002f7
62094aa8 00000000
b2094aa8 00000000
200006b0 00000001
d2000000 00000000

Any Character: Press START+L for Wario Power Up (Metal)
920966da 00000000
94000130 000001f7
62094aa8 00000000
b2094aa8 00000000
1000065e 000003ff
d2000000 00000000
920966da 00000000
94000130 000001f7
62094aa8 00000000
b2094aa8 00000000
200006a9 00000001
d2000000 00000000
920966da 00000000
94000130 000001f7
62094aa8 00000000
b2094aa8 00000000
200006aa 00000001
d2000000 00000000
DO NOT USE WHILE IN WATER!

SIZE MODIFIER CODES:

2D Character
62094a90 00000000
b2094a90 00000000
10000088 00000100
d2000000 00000000

Mini Character
62094a90 00000000
b2094a90 00000000
10000080 00000800
10000084 00000800
10000088 00000800
d2000000 00000000

Stretched Mode
62094a90 00000000
b2094a90 00000000
10000080 00001000
10000084 00002000
10000088 00001000
d2000000 00000000

Pancake Mode
62094a90 00000000
b2094a90 00000000
10000080 00001000
10000084 00000100
10000088 00001000
d2000000 00000000

Paper Mario Mode
62094a90 00000000
b2094a90 00000000
10000080 00000100
10000084 00001000
10000088 00001000
d2000000 00000000

Micro Mode
62094a90 00000000
b2094a90 00000000
10000080 00000200
10000084 00000200
10000088 00000200
d2000000 00000000

Mini Mode
62094a90 00000000
b2094a90 00000000
10000080 00000500
10000084 00000500
10000088 00000500
d2000000 00000000

Small Mode
62094a90 00000000
b2094a90 00000000
10000080 00000a00
10000084 00000a00
10000088 00000a00
d2000000 00000000

Big Mode
62094a90 00000000
b2094a90 00000000
10000080 00002000
10000084 00002000
10000088 00002000
d2000000 00000000

Bigger Mode
62094a90 00000000
b2094a90 00000000
10000080 00003000
10000084 00003000
10000088 00003000
d2000000 00000000

Mega Mode
62094a90 00000000
b2094a90 00000000
10000080 00004000
10000084 00004000
10000088 00004000
d2000000 00000000

Super Mega Mode
0217f738 00006000
0217f73c 00006000
0217f740 00006000

WARNING: GIGANTIC Mode!
0217f738 00010000
0217f73c 00010000
0217f740 00010000

SPEED MODIFIER CODES:
HOLD the Y button to & RUN in any direction. Once you start running, the speed boost will kick in!

Run Fast
4216e370 0001df00
92096b5c f7ff0800
0216e370 00030000
d2000000 00000000
4217f750 0001d000
92096b5c f7ff0800
0217f750 00030000
d2000000 00000000
HOLD the Y button to & RUN in any direction. Once you start running, the speed boost will kick in!

Run Really Fast
4216e370 0001d000
92096b5c f7ff0800
0216e370 00080000
d2000000 00000000
4217f750 0001d000
92096b5c f7ff0800
0217f750 00080000
d2000000 00000000
HOLD the Y button to & RUN in any direction. Once you start running, the speed boost will kick in!

Run Super Fast
4216e370 0001d000
92096b5c f7ff0800
0216e370 000c0000
d2000000 00000000
4217f750 0001d000
92096b5c f7ff0800
0217f750 000c0000
d2000000 00000000
HOLD the Y button to & RUN in any direction. Once you start running, the speed boost will kick in!

Run at Bullet Speed
4216e370 0001d000
92096b5c f7ff0800
0216e370 00100000
d2000000 00000000
4217f750 0001d000
92096b5c f7ff0800
0217f750 00100000
d2000000 00000000
HOLD the Y button to & RUN in any direction. Once you start running, the speed boost will kick in!

WARNING: Run at Warp Speed
4216e370 0001d000
92096b5c f7ff0800
0216e370 00200000
d2000000 00000000
4217f750 0001d000
92096b5c f7ff0800
0217f750 00200000
d2000000 00000000
HOLD the Y button to & RUN in any direction. Once you start running, the speed boost will kick in!