In somewhat of a subversion for a Slowbro, this guy's not really dumb. He can actually be pretty smart if he really puts in the effort. No, it's more that he's stubborn. Very much so. Edea levels of stubborn. He will absolutely refuse to back down once he's settled on something, and he certainly has the stamina and iron will to tire any potential opponents out.
That said, because of this, he's incredibly difficult to bring to reason at times, so yeah, he can be pretty stupid. Apparently he doesn't mind being hit either, maybe he's just that sturdy. He'll often use that, uh, talent of his to great effect both in battle and outside of battle, as he's adamant on being everyone's shield. Eh, at least he has his heart in the right place.