I'm somewhat tempted to say Lower-Mid. Normal is a pretty good type to have, and it has access to Payback and Reversal, which gives it near-unresisted Fighting/Dark coverage. Wild Charge and Outrage are good moves to have for Iris, and its defenses are good enough to take hits if it needs to. Unlike something like Throh, which is similar statistically, comes at the same time, and is Low Tier, Bouffalant has the power and coverage to destroy most of the E4 without a large amount of help. I'll wait for some other arguments before making a decision, though.
Bouffalant doesn't get Outrage. It is a move tutor for post-game (Nacrene City). However, I do agree with what you said. [1/3] Bouffalant in Lower-Mid. I used one in-game during my last playthrough and it took on Iris and 2 of the Elite 4 easily, courtesy of its good defenses and great HP, plus its sky-high Attack.
In order to understand my train of thoughts,
you'll have to put yourself in my position. You can't expect me to think like you because my life ain't like yours; You know what I'm sayin?