[Stephen304]

There's nothing stopping you, the source code is freely available to edit when you start up your instance. It would probably limit the appeal of your instance to many people and maybe some very ideological smaller instances might defederate, I doubt the other major instances would care much since only those signed up at or browsing through your instance would see the ads.

[tayo42]

I guess if I put enough effort into it I could fork it. Is it that easy though? Or not built in already