Oh, I'm not saying Thor would be bad for the job. Just that Tyr is in fact the right man, whereas Odin and Thor both behave like rash youngsters in a way. Maybe a thorough analysis of the corpus might tell us more about which are earlier stories and which are later, and thus which might be from the time when the cults of Thor and Odin were in direct competition. But I didn't mean to imply Thor was "mentally challenged", or whatever the current euphemism.
BTW, as for the other meaning of "dumb" (that association in itself says quite enough, I guess): Den lille Havfru gets to be human, in Andersen's tale, but she's then unable to speak. To me, that has a somewhat artificial flavour. Are there any other examples of mermaids or .men that can't communicate? Similar cases? (Check the reaction of the people when they first judge Frankenstein's creation).