Was Kenneth jealous?

I think it was a mixture of anger and jealousy. He seemed very resentful when Ronald and Cindy pulled up in the car together.

I worked with a guy like Kenneth, only worse. He wore his grandfather's clothes. (He told me this. He said when his grandfather died, he took all his clothes since they fit him. Or at least he felt they did.) He cut his own hair (why pay someone) and you could tell. His glasses were taped together (nothing wrong with the lenses, why buy a new pair?). And complained he couldn't get a date. Even cursed at every girl who turned him down. And would always bring up his appearance. I was like, either do something about it, or accept that a lot of girls just will not want to date you, without being nasty about it.

[deleted]

In a way he could've been. But think about if your best friend started thinking he was to good for you out of the blue. They blow you off, doesn't want to be seen with you and trashes your house. I wouldn't be happy.

You said yourself, Kenneth's understandably angry with Ronald for ditching him - consequently he'll be hoping that he's unsuccessful and/or makes a fool of himself when trying to "hang with the cool kids" and is miffed when he doesn't.
Further, I think Kenneth's motives with Patti were simply because he fancies Patti - I don't think he particularly wanted to be friends with the entire popular crowd, especially the guys, because to Kenneth they were jerks who'd trashed his house for at least a couple of Halloweens before the s*** incident. We're not shown enough though about how Kenneth started tutoring Patti to know.
I would say Ronnie's speech when he saves Kenneth's a** explains the difference in their method.
I'd also argue that it still wasn't considered "ok" because Quinton was ready to deck him for sitting there, and the other jocks made no attempt to stop him even after Cindy pleaded with them (in fact it was the blond guy who brought Kenneth's presence to Quint's attention when he expressed his dissatisfaction about it).
As for the popular crowd making a choice to become friends with or like Ronald, all of it was predicated on the fact that he had Cindy's approval - Patti/Barb (can't remember which one) even admits that the main reason she asked him out was because he'd been with Cindy.

Kenneth had every right to be angry and perhaps even jealous. There were several occasions even before Ronnie threw dog poop on his house that he completely disrespected Kenneth and his friends by blowing them off. The time when Kenneth almost got his face rearranged by a visiting fan while trying to save Ronnie a seat at the football game was one of many. It is true that Kenneth was looking forward to the "cools" remembering who Ronnie really was when he and Barb pulled up to the strip in Mr. Miller's Chrysler, but at that point you really couldn't blame Kenneth. Maybe if Ronnie had at least offered up some sort of explanation instead of just blowing him off Kenneth would have been more understanding. Hell, he may have even been happy for him.

And there really is no comparison between how Kenneth and Patty were becoming friends as opposed to how Ronnie and Cindy became friends. As Ronnie himself said in his final speech, he messed up. He tried to buy his way in. Kenneth was not trying to buy anybody.