All strictly IMO:
I am in the group that thinks defining silly is difficult.
Humor is one thing, camp is another, and outright stupidity is something else again.
Some monsters weren't meant to be taken seriously, or at least I thought so when I originally read about many of those that have been mentioned so far or that were referred to by links.
I think that the Flumph, Flail Snail, Giant Space Hamster, and their many cousins were just there to laugh at. I never used any of them. They did cause some irritation on my part, as I felt they cut into the overall value of the products i was buying, but it was only minor. I wished at the time that they would assemble that sort of thing into specialized products for those who liked and would use them, but I never have been able to cast wish.
Did anybody actually use the Iridium Dragon? Bigger than worlds, it flew around attracted to Monty Haul worlds, where it would eat them unless all the world's artifacts were sacrificed to it. I laughed when I read it, and it was certainly silly. It was definitely a joke.
I can imagine a game based on humor, camp, or over-the-top ideas could make great use of things I think are silly.
Perhaps the topic should have been titled: What is too silly for serious D&D?
However, beyond just simply monsters, I didn't like what I felt was too much camp being injected into Spelljammer and Planescape. It was difficult for me to swallow at the time of their release, although I like both settings more now than I did in the past. (So age and current world-view/outlook can influence what is, "Too silly.")
I'm not sure many would agree with me, but I felt Cyric, Kelemvor, and the other girl (I think her name was Midnight, I can't remember) who all became greater gods definitely fit into the stupidly silly category.