I agree about Killer Clowns. I didn't see what the fascination was.
While we're thinking about Baron von Munchausen, honorable mention (at least in my book) to Time Bandits (somewhere treading the ground between sf and fantasy, but I liked it!)
My dad loved Tremors, but I thought it was kind of stupid, but a lot more fun than Killer Clowns. too...a lot of bad horror stories are told in an sf motif....
And then there was My Science Experiment....
There was a Spielberg SF spoof in which the superimposed cartoon aliens wound up killing everyone (including Jack Nicholson in his role as President of the US, if memory serves). That sucked. There was an amusing movie in which some alien gang that couldn't shoot straight tries to conquer the Earth, but alas arrives on Halloween, so no one will take them seriously. Can't remember either title.