As an example: That "dumb girl" in Borderlands The Pre-Sequel, Janey Springs. Her whole backstory sounds like a reference to an old court case, of a mother whose daughter was eaten by dingoes. If you check her old camping site, you'll see a torn teddy bear there, and Janey is writing nursery rhymes - perhaps as a homage to someone she lost? No, fandom, the one Flamey Junior ate wasn't Janey's lover.
Also, she's not thin and "hot" because she's vain. She's been stuck for months in a place surrounded by lava, huge man-eating rock creatures, and bandits. She's starving.
In short, Borderlands' humor is generally "silly" or "dumb" only if you're paying more attention to shooting things than the details. See also, Tiny Tina. "Annoying" and "stupid" unless you really paid attention to her backstory. Girls' got plenty of excuses to be missing a few screws, and her tea party mission was one of the best things in BL2 for me.