No one hates fun. People just don't want to see something that goes against what we have been told in the story. A perfectly reasonable thing to want. In the story, trolls don't have mother or father figures because their parents are animals that aren't even capable of regular speech (until they were prototyped at least). Vriska also says that descendants look up to their ancestors as legends and try to follow in their footsteps normally. The concept of parents, who are the same species and act like a human mother and father typically would, does not exist in their culture.

No one is talking about whether or not an Ancestor can take them on in a mentor/student relationship, the topic is people who literally have Ancestors act like parents to the younger trolls. That is what's wrong, as defined by canon. Personally, I can see something like what you described. But someone acting as a mentor figure =/= someone acting like your parent.