Speaking only of games I’ve actually played on the Deck:

Cobalt Core has been very charming, I’m surprisingly enamored by it. Excellent roguelike deckbuilder mechanics too, though you may find it a bit easy to “break”.

Celeste is a classic. Tough as nails platformer with gorgeous art and music and themes about overcoming anxiety and depression.

Dicey Dungeons is another roguelike (kinda) with super charming visuals and audio design. There are a ton of different gameplay mechanics in this one and the lore’s cute.

Hi-Fi Rush is a combination of Devil May Cry / Bayonetta with a rhythm game, and the art style is cartoony and fun and over-the-top.

Slime Rancher is another wholesome feel-good cute game but it’s pretty light on the story.

Stray is a kitty game where you’re an adorable kitty kitty. It’s recently received a patch to fade to black earlier in certain fail states if you don’t want to watch the (already incredibly mild) cat game over animation.

The Stanley Parable (Ultra Deluxe) is a surreal adventure with branching paths and a snarky narrator dueteragonist. It’s funny and engaging.

Undertale has some heart wrenching story moments but is generally fun and charming in tone - it’s a game where you don’t have to kill anything, but if you do kill things it can get darker.

Emulation-wise:

Super Mario RPG Remake should run well on Deck once it officially releases and the emulator devs can release the patches that they totally aren’t already working on. Super charming game and the remake is actually fantastic!

In fact, any Mario RPG is likely to be mostly good vibes, in contrast to a lot of other traditional RPGs with their angst and grief and high personal stakes and death. Thousand Year Door, any of the Mario & Luigi games, they’ll all run perfectly on Deck. Mario Wonder is of course story-light but the definition of a whimsical good time.