I don't know as Mono is any less native than .NET, they both have to JIT the program's IL, and the .NET APIs don't really expose much of anything that's platform-specific so there isn't really a "native" platform.
Mono even had AOT compiler with capabilities that are only now landing on Native AOT, and are the underlying architecture for what made Blazor possible in first place.
The real reason to use KeepassXC over Keepass is because it's much better! The UI is better in almost every way.