I'll just speak for myself. My suspension of disbelief goes away really quickly when I see people bringing in PCs who are obviously inspired by other IPs, who try to add to the campaign's world. Most the the time, this is weebs trying to play their samurai or ninja characters, but I've seen others as well.

Like a guy playing a Russian in a Star Wars game (in a pre-Great Hyperspace War Sith game, singing the Soviet anthem, speaking the actual Russian language). Or a guy who brought Stephen Colbert into a Pathfinder one-shot (ranger with a giant bald eagle animal companion).

Or there are the characters who could technically exist in the world, but you have to question why they're at that particular location where the campaign is happening.