It's not necessarily meant to be a straight up joke, it's really just Robin reacting to something she found slightly surprising/unexpected regarding Nancy.
You can tell from Robin's tone and enthusiasm when she delivers the first "You have a Tom Cruise poster!" line, that she herself is a fan of the actor.
Stranger Things season 4 takes place in March 1986, a couple of weeks before the original Top Gun would be released. Tom Cruise was not yet the action star in the cultural perception, but he was "already a heartthrob due to his now iconic underwear dancing scene in 1983’s Risky Business" (source).
Robin's tone changes to a teasing one when she delivers the line for the second time. She's like: Wait... you, the always proper and mannerly Nancy - you have a poster of Tom Cruise the heartthrob! For Robin, it's a little glimpse behind the facade Nancy presents to the world.
Nancy recognizes the tease, and she's perhaps feels slightly embarrassed (while being simultaneously annoyed), so she quickly dismisses it by saying something to the effect of: Oh, that ... yeah, that's an old poster, whatever infatuation I had back then, I've grown out of it...
The actual line is much shorter, because Robin proceeds to go through her stuff, so Nancy doesn't have time to dwell on that.
"That's ... [cogwheels turning] old. Can you please not touch anything?!"