What does Sir Percy do to confirm the common assumption that he is stupid?

Sir Percy confirms his stupidity by allowing his wife to ridicule him.

But then Blakeney was really too stupid to notice the ridicule with which his wife covered him, and if his matrimonial relations with the fascinating Parisienne had not turned out all that his hopes and his dog-like devotion for her had pictured, society could never do more than vaguely guess at it.