OPs comment claims there's not
a movie of a girl still in her prime dating the nice guy
Don't misunderstand, I think it's an exaggerated claim, an over generalization with possible counterexamples, but it doesn't seem to miss that far from the mark.
If you judge simply by superficial appearances, you might call 'Knocked up' an obvious counterexample, but there's probably enough girls who think Rogan is cute and boys who think Heigl is a bitch, to consider it a bit more.
Beside the rather insulting and sexist phrase 'in her prime' (which for many 'niceguys' no doubt skews young and means maximum sexual attractiveness to them), OP has a bit of a point (if only by luck), namely stuff like this and this (sorry it's buzzfeed). The so called hollywood age gap, especially for serious or general audience films. Look at Harrison Ford, or Liam Neeson and their pairings (No hollywood or mainstream actor is a nice guy, or 'niceguy', they're already exceptional, possibly charismatic, and successful, in that sense it's always false. I'm not sure if that doesn't moot any claimed counterexample right away, but supposing it doesn't)
The few films that do prominently feature an 'ordinary' or meek and mild mannered men, or boys, who date girls 'in their prime', tend to be teenage comedies, which you seem to refuse to consider as possible proof of OPs claim.
An Ordinary Man is about a nice meek man, but it's more about adult dysfunction than dating IIRC. In Horrible Bosses 2, the hot woman 'in her prime' rapes the meek man who then wants to murder her, he doesn't seem like a nice guy, and again a comedy. Michael Cera in films (like Scott Pilgrim) he's almost always a jerk (Youth in Revolt tries to play with this trope), or Aaron Taylor-Johnson as Kick Ass (is he a nice guy?). I suspect the OP might argue those male leads were never, or by the end aren't, ordinary nice guys.
It's part of bildungsroman for teenage boys, they have to aspire to dating the most beautiful unattainable girls and women as trophies, proving their worth and arrival as adults (probably not so dissimilar to girls and disney with princes). Films from Risky Business, The Girl Next Door, Superbad, Porkys, The Breakfast club, Weird science, something Jumpstreet, Horror movies, and many many more feature typical popular girls 'in their prime', and jocks and nerds and all those high school tropes.
Let's look at Breakfast club, IIRC much of the crux of the drama is the popular girl getting to know the 'bad boy', look at the famous last scene, it wasn't of Anthony Michael Hall. He played Rusty as a sort of pathetic nice guy in vacation, who doesn't date a girl, while Christie Brinkly teases his father (because that's funny!), and in Weird Science, he and his friend are so 'thirsty' they have to create their own masturbatory creation, the premiss of the film, hardly nice and ordinary. Look at Easy A. Emma Stone is 'in her prime', but only fakes dating all the outcasts and losers as a sort of high school social experiment, when she finally really dates, it's with a character that's been redeemed or elevated out of 'nice guy' status, if they were ever really one to begin with (handsome athletic looking, but not an obvious jock or nerd, 'humble' enough to wear a funny costume, ideal high school boyfriend matierial). Risky Business, Tom Cruise ends up as a pimp, hardly a 'nice guy'. Maybe nice guys are Cyclops in Xmen, or Cyril in Archer, but they're not leads, but for comic effect, sometimes the straight man.
I get the impression if a lead character was a really nice guy in a film, it would be very boring (there isn't romantic drama without problems which nice cause tend not to cause or resolve) or if they're 'nice guys', they're really insufferable, disgusting or despicable if maybe still attractive (like in Archer, Bad Santa, Mall Cop, James Bond, and many many more).
Knocked up doesn't seem a convincing counterexample from what I've read, since Seth Rogan is Hollywood 'nice', or someone can just claim Heigl was 'a bitch' and 'out of her prime' (as a teenager) if that makes sense. Maybe he or Heigl just aren't convincing in the roles or it's not a very good film. Maybe his character develops out of being a nice guy by the end or was always a jerk. I haven't seen it and don't plan to. The closest other example I can think of is, Young Adult between Therons character and Patton Oswalts (which I haven't seen only read about), but even there she's supposedly desperate 'out of her prime'.
Maybe I'm obtuse, but I can't think of a good dramatic film where the lead actress isn't noticeably younger than the actor. A film about losers losing doesn't seem a formula for success with teens. Wait a 'nice guy' film I can think of is 'Elephant' but IIRC they didn't date any girls in it. If some slutty girl had 'taken one or both of them' for the team', she might have saved a lot of lives at little cost, but that's very cynical.
In the words of Louis CK, no one dates down, maybe sideways but not down. Women seem to suceed in hollywood more easily if they're beautiful (e.g. Margot Robby and Daisy Ridly at the moment), while for men, it's if they're funny, handsome or can act, or some combination. If women dates someone 'successful' (better if they're also handsome too, but it doesn't seem required), they're dating 'up', while for men it's primarily if they're beautiful.
He is saying that this is not a feature commonly seen in current entertainment, but it totally is. Is it ridiculous? Probably. Is he wrong? Absolutely.
I'm not sure I can agree when you look at any particular film.