Pokémon Masters Arena (not to be confused with the 2019 mobile game Pokémon Masters) is...a bunch of generic minigames. That’s the most flattering description it could be given.

Unused Graphics

Please elaborate.

Having more detail is always a good thing.

Specifically: ”more unused graphics than what’s listed”?

While there's more unused graphics than what's listed, these are the most interesting.

sxx_ynYes.smk and sxx_ynNo.smk are leftovers from Blue's Clues: Blue Takes You to School.

ma_host01.smk and ma_host02.smk are leftovers from Care Bears: Let's Have a Ball!.

Temporary graphics for Veggie Tales Jonah: Kahlil & the Camels, which seems to be an early name for Jonah: A VeggieTales Game, and a password entry.

Leftovers from Jonah: A VeggieTales Game.

Unused PLAY GAME and UNLOCK LEVEL buttons.

Unused completion screens with a flat blue background.

Unused Music

The ending song to Blue's Clues, also leftover from Blue's Clues: Blue Takes You to School.

Unused Sounds

A twinkling sound from Care Bears: Let's Have a Ball!.

The ICQ "uh-oh!" sound.

A creepy, low-quality error sound.

The Windows XP error sound.

Voice overs leftover from Jonah: A Veggie Tales Game.

bmpinfo.txt has a build date for the game.

BMPINFO v1.00 Build: May 7 2003 17:05:09 Copyright(C) 2003 ImaginEngine Corp. All rights reserved.