We love the Steam Deck, and are happy to say that the game (and demo) works great on Steam Deck. So while we wait for the official Steam Deck verification process, here's our own small seal of approval:
ːsteamthumbsupː ːsteamthumbsupːːsteamthumbsupː
Moomintroll: Winter's Warmth is on Deck
Source: Steam Community (opens in a new tab)