You’d be buying the domain name, not the lemmy instance. So if you wanted to replace every post with whatever, you’d first have to set up your own server and create a new lemmy instance for your newly purchased domain name. You’d have to take extra sets then to mangle your lemmy instance such that it didn’t show the real posts but instead showed what you wanted…

In the end that technical work would be more expensive than the domain name… and probably not worth the effort for a short-lived and mean-spirited joke.