I found the 2 Mencia episodes fascinating. Any sympathy I may have felt for Mencia evaporated when toward the end he claimed he had friends who wanted to beat up the people calling him out for joke thievery, but Mencia told them not to do it. What a humanitarian.
On the joke thievery bit, Maron made the good point that with the really hacky jokes (black people drive like this, white people drive like this), a lot of times there's nothing particularly original about the jokes. They may as well be public domain.
There's also a lot in the delivery of a joke. Somebody with a great joke who sucks on stage is never going to get too far. Some people have more talent when it comes to connecting with an audience. There are 'comedians' who don't particularly have any jokes, but seem to do well on stage. The immediate example coming to mind is Charlie Murphy. For whatever reason I was watching his special, and he was engaging and entertaining, but when there were jokes, they were really no great shakes.
All that said, it was pretty clear Mencia never writes or even tried to. I suppose he did pay people (or people were paid) to write for his show, which was at least fair, but early on he was pretty clearly just stealing stuff. Even aside from the thievery he comes off as a really horrible person, his confessional contrite bits sound like they come from a bully trying to get some sympathy.