I haven't seen the show in several years but I thought it was only the pilot and the Growing Pains episodes where her character was normal? I seem to (vaguely) recall that Cindy was originally more like Wendy, and Wendy was the "ditzy" one. Then after the initial episode the characters basically traded places. It's pretty common for a show to change a character, or even change actors, after the pilot.
She never struck me as being very similiar to Kelly Bundy though. Cindy was kind of a "more heart than brains" type of character while Kelly was a trashy, shallow airhead (not that I didn't like that!)
If I'm remembering it all wrong then it's Warner Bros. fault! For some reason they enforce the copyright more heavily on this show than any other show they own. They won't let it live for 2 seconds on Youtube or similar sites, yet they also seem to have no interest in discussing any kind of syndication deals, app licenses, Netflix/Hulu availability, or even an outright sale to another distributor. No wonder the show is totally forgotten nowadays, it's almost like WB wants it that way.
I am watching the spin off episodes on Growing Pains now. Wendy and Cindy come into the classroom looking for their dad. It is Cindy who is the stronger of the two girls. She ordered Wendy out of the classroom and to follow her. She spoke with strength and confidence.