The entire "Heart Smashers" episode bugged the crap out of me. Why is Fenton suddenly Brendon's friend when it's been firmly established that Brendon can't stand him? Why do Jason and Melissa act like this state of affairs has been going on for some time and is not unusual? Why does everyone decide that Fenton is the only one fit to play the part of the possessed stowaway when all he's done thus far is goof off and screw up the movie? When did Paula become such a pushover? How the hell do you blow up your pecs? Muscles are not balloons. Granted, it's a cartoon, but it was still quite subdued and realistic up until that point. It's not a "wacky slapstick" cartoon, so this was extremely jarring and just felt like a cheap excuse to show Coach McGuirk with saggy breasts.
You've never had a kid you couldn't stand "tough it out" in a circle of friends and slowly get accepted? I will grant that they usually just become the whipping post and get traumatized instead.
Also, Fenton clearly has some sort of personality disorder which makes him completely oblivious to just how irritating he is to others.
Fenton's character got seriously derailed. When he's first introduced, he's the archetypal Spoiled Brat, always getting what he wants and never being happy with it, and his mother always caves in and allows herself to be walked all over. At the end of the episode, McGuirk tells Fenton off in a Crowning Moment of Awesome, and his mother finally gets her act together and puts him his room. So, OK. His mother's grown a spine now, he's going to start getting some discipline, it's fair to expect a few changes. However, when we next see Fenton, he might as well be a different character altogether. Suddenly he's this hyperactive, disturbing momma's boy who enjoys doing chores and is obsessed with making movies with Brendon, despite having previously been infuriated with him for his extremely unflattering "video-film". Also, his mother's become this crazy puritan who doesn't allow TV, serves plain rice for dinner and makes Brendon work in her yard in the rain. Not one iota of this is explained or even noted as strange by any other character. Possibly a case of Negative Continuity, but considering that the show has otherwise demonstrated well-written and consistent character development, it still wouldn't make much sense.
A bit of Fridge Brilliance to me. When McGuirk inspired Fenton's mom to start acting tougher, she did. And she went a little to far, turning into an over-controlling mother (perhaps out of necessity). Fenton just fallowed suit, almost seeming to worship her and becoming the spineless one, though only in front of her. He's still a brat to everyone around him, just not his mom.
Although I'm fine with either version of Fenton (actually, I slightly prefer his later characterization), his change is rather abrupt. It's all the more jarring in that his mother in later episodes is depicted as pleasant and easy-going as ever, if not more so.
My view on it was that, given McGuirk's rocking Moment of Awesome (as mentioned above) Fenton's brain simply broke, arguably being one of the few positive things McGuirk has ever managed. The tweak in Fenton's personality comes from a combination of his mother growing a spine and Fenton's newfound fear of reprisal. If his mother never kept him in line before, even a mild addition of discipline might send him into a completely reversed reaction, becoming a sniveling momma's boy.
Again, Fenton seems to have an underlying personality disorder—though, to be fair, that's true of a number of Home Movies characters.