Luna isn't even close to being among the dumbest Holos. She's actually probably one of the smartest, considering how she's able to stay in character even when doing things like talking about her surgery or playing the piano.
Baqua is the dumbest by far, practically in a league of her own. Nene and Gura are probably next but they do play up it up a bit, after all Nene's a resilient girl who kept sending in applications until she got accepted and Gura already had VTuber success before Hololive. They both know how to play up the part of a baka, but a bit of that is probably real.
Aqua though... Does no one remember this video? (Subs on CC)
https://www.youtube.com/watch?v=wUS7lIOqTPw 
