It’s not historical but medieval fantasy settings generally tend to be based around medieval European tropes. I guess I just assumed your problem was with period accuracy because there are variations in the accents. Karlach has a working-class accent, Asterion is posh, Shadowheart is middle class? british.
It could be more overt, I suppose, or incorporate more than just british regional accents, but I feel like they made more of an effort than you’re giving them credit for. I really disagree with any of it sounding like a theater troupe, though, save maybe Dame Aylin but that’s in-line with her character.