Was the Spaniard really dumb or not? If yes then why did Huck told Mr.Jones that one of the man was dumb?

No he is not deaf and dumb. The Spaniard begins to speak and they immediately recognize the voice as belonging to Injun Joe! When the boys first see Injun Joe, he is wrapped in a serape ­ a traditional Spanish article of clothing ­ and disguised as a deaf and dumb Spaniard. We begin to notice that with his villains, Twain seems to represent an element of the exotic and the foreign. Whether dressed as Injun Joe or a Spaniard, he is never mistaken for a typical inhabitant of St. Petersburg, but rather is presented as out-of-the-ordinary.