This show jumped the shark in a special way.

Frankly Urkle made this show. I watched the first 2 seasons on DVD from Netflix a few years ago and I am sorry. But the first season was kind of dull without Urkle.

Those dummies (if you mean Stevil and Carlsbad, which I guess you do) only appeared in two Halloween episodes, so I would say that they don't really count for this discussion. This is not the only sitcom, that had some weird and creepy Halloween episodes at times. So I don't think that Stevil and Carlsbad necessarily are a jumping the shark moment.

And as for the sci-fi elements... I'm probably in the minority about this opinion, but I don't really mind them. I found the character of Stephan so boring though. I'm trying to think of him as a welcome opportunity for Jaleel White to be on the show and still act more like himself. But I could never ever warm up to Stephan, because I was much more interested in the cute and nerdy Steve than in his lame but supposedly "cool" alter ego. So let's just say that I totally share Myra's taste in men. ;)

I think the Jump the Shark moment happened sometime when the kids were in college, and the parents became less relevant for them. In fact, the really wacky stuff started happening in the very last two seasons, when the show's ratings started to really go downhill. A pity. Some of the most interesting adventures happened during that time.