I think that the main issue is that the secondary characters are very one dimensional. This is hardly surprising - it’s a short cartoon, not the kind of format for extensive character development. So the minor characters are basically stereotypes.
I think that it would be better to have more minor characters, so you could have a greater range of them. It wouldn’t matter of the Simpsons was just one series, but given how long it has run, they could have expanded things more.