This is definitely an interesting question. I'd say that a deaf or mute person can be tested and their lack or inability to hear or speak would not have any influence over their results. BUT it would greatly depend on the tester and their method of testing this person....
To say this a different way. An eye doc can do a pretty accurate eye exam for glasses on a non-reading child if that child knows what a picture of something is. Instead of asking that child to say what letter they are able to see they ask them to tell them what the pictures are. So they see a star instead of an "A" or "B".
This eye doc has found a way to accurately gauge this non-reading child's vision, a way around something they can't do. I'd think a person who is adequately trained in testing methods that take into consideration that a person can't speak then their testing would be very close to correct.
I'd want to specifically know their qualifications and how they gauge that person's intelligence.
For instance, the age of the child plays a huge part in this method. If the child isn't able to say "Dog" when the person testing/evaluating asks them to name the animal in the picture and they don't allow them to point to the word or some other means to signify they even heard the question then they don't have an accurate method.
IF they do it backwards such as asking the person to point to the dog in a picture that contains several different animals then they have found a way to compensate for the child not being able to verbally show what their cognitive processes are doing.