Of course, the reaction to Biden being president in 2021 would be, “Good for him, he finally won.”

One plausible avenue you could take is that 2021 Marty is an influencer/youtuber, and uses his film/marketing skills to do something cool. There are lots of teenagers today with filmmaking skills that would rival professionals in the 1990s, and the That’s one way in which someone in modern times could inspire 90s kids to all think he’s the greatest.
He could invent the modern meme.
I canhaz chezeburger isn’t cliche yet. Nor is keyboard cat.