That's because performance is a criteria for getting verified:
default configuration: the game must ship with a default configuration on Deck that results in a playable framerate.
Source: https://partner.steamgames.com/doc/steamdeck/compat#DeckCompatibilityChecklist
There is no "target framerate" though, so what's considered "playable" differs from tester to tester.
Yes, Mono is used by Wine to support Windows .NET applications since it's a) open source and b) contains support for Windows Forms and other Windows-only APIs.
They can't ship the regular .NET framework by default for licensing reasons but it can be installed with winetricks to replace Mono, which is sometimes necessary for compatibility reasons.