Pretty sure this same list is all over the internet but doing some reverse engineering and thought to dump here straight from the game's code

List of Cheat Codes

PINHEAD Pin Head Mode

PATWELLS Beat This!

UGLYMUGS Team Photo

MOWER Invincible

TRICKSTER All Stunts

XLURIDER Transparent Rider

ROTCOLS Odd Color Mode

MIDNIGHT Night Mode

INVISIRIDER Invisible Rider

SHOWOFF Stunt Bonus

WHEEEEEEEE Down Hill

IMGOINGNOW Debug Mode

YADAYADA Mirror Mode

BLAH BLAH Big Head Mode

I've wondered if the PATWELL cheat does anything. It didn't feel like anything changed when I first tried it out, but I'm willing to give it another shot

PATWELLS is apparently a reference to a QA Tester for NOA who worked on Excitebike 64: https://www.mobygames.com/person/59394/pat-wells/

The IGN article with the code has this bit:

Use the name of Nintendo's Excitebike Master (Pat Wells) to make the game a lot harder: to do this, enter PATWELLS as a code.

I believe it makes the CPU difficulty another notch higher than Professional when enabled (presumably, this is the same effect the code "GAMEYARONI" has in the NTSC-J release of the game).

In the Japanese release, the codes are given to you when completing championship rounds. The Hi-Res mode setting in the Options menu is replaced with a Cheats option.

Codes for the Japanese release below:

・ KIDSRIDER (Beginner / Bronze) The rider's head gets bigger.
・ MIDNIGHT (Beginner / Silver) The course will be at night.
・ PINHEAD (Amateur / Bronze) The rider's head becomes smaller.
・ BIKEDAKE (Amateur / Silver) Only for motorcycles.
・ 10TENTORE (Amateur / Gold) Soccer will be 10 points first.
・ HENNAIRO (Pro / Bronze) The color of the course changes.
・ HYAAAAAAAA (Pro / Silver) Hill climb goes down.
・ TRICKSTER (Pro / Gold) You can use all stunt moves.
・ GAMEYARONI (Pro / Platinum) Computer becomes stronger.
・ MUTEKIDAZE (Pro / Match Race) The rider becomes invincible.

Challenge to the Past did a run with this code active as well as not using any boost.