I'm now thinking back to the one Xanth book I read as a kid, and having a lot of realisations about how many really sexist and rather not-okay ideas were in it, and... yeesh.
As to Thaco the clown... I can definitely see why the people it's lampooning would take offence to it. But it feels to me more like self-referential tongue-in-cheek easter egg than attacking a group they feel is "other". Especially since the majority of their current audience probably doesn't even know what thaco is, and the reference is going to fly right over their heads. Still, considering he appears to be an entirely negative character based on an entirely negative stereotype... yeah, probably not something I would have included.
No one is saying that people shouldn't be offended. People should feel what they feel. We derive meaning and build our identity from a myriad of things and experiences, and that may cause us to atribute a joke directed at something we like as a joke directed at us. That is a natural human response. That said, I think it's valid to differentiate when a joke is pointed at a thing we like (even if I as a person derive identity form it) from when a joke is directed at a person or a group.
The community agrees: Thaco the clown is good, old fashioned, corny fun. On a traditional scale of 1-10, I would give Thaco the Clown a rating of -5. That's a good thing. That translates to a 19 on a modern 1-10 scale.
Remember the old 4e joke about the troll getting pooped on? I mean, it was a pretty obvious send up of internet trolls, and not meant to be a broader joke. But, whoah, did folks take that one personally.
And, here we are, ten years later, and a teeny, tiny joke, a bit of a send up that's not even really mean spirited, becomes a major issue. Sorry folks, but, if this joke gets your back up, perhaps a little self assessment might be in order. Why does this joke bother you? Otherwise, we're back to people losing their poop because WotC mentions cloud watching.
Just a thought, but they might be poking fun at the RPGPundit, one of the consultants they removed from the 5e Players Handbook. He checks off just about every box, including always having a pipe in hand.