DOA6 has characters that are strong for various reasons, but i personally wouldn't say any of them are dumb, i think the closest a DOA6 character has ever been to being dumb was pre patch Nico, but besides that nothing else comes to mind, Tina might also be a contender, provided she's not fighting a character with a i11 6p, she can be pretty dumb, even with the normalized throw damage
Dumb as in low IQ? Or dumb as in gameplay? Because Kokoro, Hitomi, Jann lee, Hayate, Bass Honoka, Eliot and Brad Wong really seem they’re not that bright tbh.
As in gameplay, Nico, Diego, Phase 4, Bass, and Kula.
Hmm... Honoka seems like an idiot. But yeah, with regards to the roster balance, no character would be considered "dumb" (aside from maybe Phase-4 & Kula). NiCO & Tina are more equalized now from how they were early on. That's what I prefer about DOA6 over DOA5LR, the roster balance is a lot better. DOA5 had dumb characters littering the roster (and TN correctly opted to remove them).
"Dumb" can mean a lot of things. If you're talking about the characters that require the least amount of effort to reach competent performance, Diego is your guy hands-down. Honorable mentions: Kokoro and Hayate