well, some of it is sarcastic exaggeration to make a point. like my fat, bald jetpack Superman example.
but here's a much more realistic example.
let's say the new Peter Parker starts out as a blond, confident, popular HS baseball player who then gets bit by the spider. After he becomes Spider-Man, he starts studying up about spiders, which leads to increased interest in biology and chemistry. Peter realizes science is actually his true passion, and he starts neglecting his athletic pursuits. He also starts losing friends from the "jock crowd" and starts making friends amongst the geeks.
To go with his newfound interest in science, Peter also starts changing his appearance to make a "clean break" from the Peter of the past. He dyes his hair brunette. he starts wearing glasses, and he starts dressing more conservatively.
by the end of the film, he starts to look and act more like "classic" Peter.
now, would you want to see a Spider-Man movie where Peter goes through that character arc?
I certainly wouldn't. Cuz it doesn't even sound like Peter Parker at all.
it's a story arc that doesn't make sense for the character, because it makes him start out in a position where he's the total opposite of what he's supposed to be.
it's essentially a reinvention of the character - to the point where it feels like a totally different character.
And if such a scenario isn't acceptable for Peter, then why is it ok for something very similar to happen to MJ?
because with the direction they seem to be taking MJ, they are giving her a story arc that doesn't fit the character, making her start as the total opposite in terms of looks and personality so that she can "transform" into the more familiar and classic MJ by the end.