Decompilation is not the same as source availability. Any binary can be converted into source that will compile back into a (nearly) equivalent binary. Whether that source code is economically viable to use in some derivative work is an entirely different question. It might be easier for .NET binaries to be decompiled but the point still remains that machine generated source code is not the same as human generated source code.

What does the output source look like? Is it legible?

Would decompiling something like paint.net and attempting to compile the out source with Mono (for use on Linux) be a viable pursuit?

I am unfamiliar with the nuances of Mono vs Dotnet, particularly in graphical applications