My point of view on that whole part of the movie, is that Kim was hoping Ron would say something about his feelings from the time when he said "I think I know what this is about", when he has his petition for Bueno Nacho.
Notice how she seems to get her hopes up, followed by disapointment when Ron apologizes for not beating Dr. Drakken.
On the phone, Mrs. Possible pretty much tricks Kim into saying that it is because of the food chain. And when Kim does say it, she realizes what she has said and quickly says "No!"
The reason Ron is not an acceptable date, or 'boyfriend material', is because he would be going to the prom with Kim as "best friends",
not in a romantic capacity. that is the problem. Not that it's Ron. The part about what Bonnie said that really got to Kim was not the food chain part, it was the you don't have a boyfriend part. She knew the whole time that the 'food chain' part was a stupid thing, but I suppose focusing on it was her way of (not) dealing with the real issue.
In the next scene, Monique almost has her talked back into liking the idea of going with Ron, until she sees him on his sticker covered scooter. too much "child like charm"
When she sees Ron say "I'm losing everything I've ever cared about!", she knows what he's talking about. so she goes to his treehouse to talk to him. She tries to smooth things over with Ron and cheer him up,
what does she really say?
"it was fun! when we were kids, I mean"
"maybe something more than the Bueno Nacho sitch is bothering you..."
"We're not in Pre-K any more, time to grow up"
Those lines can mean one of two things, in my eyes.
1. Kim doesn't want to be friends with Ron anymore, (it was fun when we were kids)
2. Kim doesn't want to be
just friends with Ron anymore. -but she wants Ron to step up and be clear about his feelings, now, not when he was at camp wannaweep
Then the very next scene, Eric asks Kim to the prom- her reply? a nervous "oh.... K.."
Not an excited Yes! or anything like that.
Then of course you know the tied up at Bueno Nacho scene- (my personal favorite, possibly of the entire series) and what happens after that.
I don't see Ron taking advantage of someone on the rebound, I see a movie where Kim is hoping Ron will tell her that he likes her romantically, even pushing him a little. (time to grow up)
Does that mean Kim didn't like Eric when he was still good? no. But he certainly wasn't the only guy on her mind for most of the movie, or even the main one.