RPGMemes

It could be, but it doesn't have to be. It all depends on the characters involved.

The DM can referee in the dispute but they (usually) can't say your character would or wouldn't believe something.

A good DM might ask you to in-character justify your bias, for example. They're also supposed to listen if you say "I don't trust Count Fuckface on account of him having a history of being a Fuckface and also he's standing over a cooling corpse with bloody hands."

If a player is metagaming that's a separate problem from their character being biased, or having a reasonably justified suspicion or whatever.

Or never, ever being interested in the creepy player's character because they don't like their vibes.