Yeah, I can understand finding a tone that bounces back and forth between very serious and quite silly jarring, but for better or worse that's how the BG series has always been- this is a series where, in previous games, stories about the children of the dead god of murder co-exist with a side quest about collecting fancy pantaloons and jokes about miniature giant space hamsters.
It's also, as others have said, a very accurate representation of the way every tabletop D&D game I've been part of is- we have a serious story with important goals, but also the half-orc barbarian keeps trying to seduce the dragon he's fallen in love with by bringing her hacked-off parts of monsters he's killed as presents.