Knoxville was always the talent and idea man. He needed a cast of likable, funny or interesting misfits around him for it all to work.
Bam was always originally bait for the MTV teen girl demo. He was a stand-in for the cute skater boy they had a crush on at school. Im sure MTV execs loved his aspirations of being a lead singer for a goth-boy metal band too. However, his pranks and interactions with his much more personable parents ended up being far more likable and entertaining than his cliche rock star preening.
Bam’s parents /family are what made him a star, if not for them he would have just been the “cute teen skater/ boy band” of the ensemble cast.
Though I was in the teen girl demo that Bam was meant to attract I also thought he was the worst of the bunch. He seemed like a vain, rather dumb, douchebag who’s favorite pastime was looking at himself in the mirror or on video. Everybody in the production seemed to like his parents way more than him for a reason.
I’m sure Knoxville wasn’t crazy about him being in the cast but was savvy enough to know he needed to include a dreamy teen skater boi for MTV. Along the way they realized the teen/parent pranks were the gold that could be mined.
The thing about being the “cute teen boy” on a TV show is that it doesn’t last and Bam aged like milk. All his own merits were shallow and didn’t age well. The rebellious teen schtick does not work on a fat 40 year old man, even if he still behaves like a shit for brains sixteen year old man child.
I like shitting on Bam as much as everyone else these days but I don't think saying Bam was only taken on board to Jackass "by" MTV for essentially sex appeal is entirely accurate.
Jackass at its core was a Knoxville and Jeff Tremaine idea and formed around the skate and alternative culture magazine Big Brother (Pontius worked there as well).
Jeff Tremaine found out about Bam and his friends through the CKY tapes and reached out to them, before MTV was even involved.
Bam was known for some basic stunts like jumping off hotel balconies into pools and of course his skating stuff, but it was the CKY movies that made Tremaine try to get Bam and the CKY crew involved. CKY had stuff like wrecking a rental car, the guys pranking each other, as well as music and skate material generally shot guerilla-style, which all lined up with what Jeff Tremaine and Knoxville were trying to put together. Bam and the CKY crew were seen as fits for Jackass because CKY was essentially a proto-Jackass anyway, conveniently.
At the time I think attracting a teen girl demographic was the last thing on their minds, it was just finding people who could contribute to the stunts, skating, pranks and comedy.
Bam was involved before the show was even pitched to the networks and MTV fought over it with Comedy Central.
So, Bam was considered and even involved before MTV took it up or it had any real identity other than some side project of the magazine and it was on more than his "pretty" face (imo, he only really looked good around Viva La Bam. Pre-Jackass, CKY1 or CKY2K he was pretty dweeby - there's that one image where he looks like a teenaged Steve Buscemi that comes to mind too).
Funnily enough, I think Steve-O and Preston were the two to get involved
after MTV had picked up the show. So if MTV was trying to appeal to a demographic like teen girls, they sure brought on two odd choices..
Edit: For shits and giggles, found this.

Look at how fucking young they all are, jesus.