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.

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

We know how much we all love ads. Let's have more of them!

I see absolutely no reason why an instance might not decide to fund itself on local ads. I see no reason why you couldn't choose an ad-supported or non-ad instance

Forums used to be ad supported, nothing particularly wrong with being ad supported. Problems occur when your investors expect 10x return on something ad supported.

But to have something pay for it self. I'd rather not lose money on it

Actually, there is something wrong with ad supported platforms. Advertisers start imposing restrictions on the actual content, or the owners of the platforms enforce restrictions pre-emptively so it never arises.

I was on a forum, and the kind of language and even topics for discussion were severely restricted, partly because of advertising.