It is safe to say that Dr. Cass's remarks would elicit a very wide range of responses from Muslims, and almost none of them would be positive. This is because Muslims are a huge and diverse group, but one of the few points on which they all agree is that they respect the Prophet Mohammed.
Emotional responses would vary between anger and indifference. More liberal Muslims would say that Cass's opinion has no effect on reality, and does not matter. Others would view these remarks as blasphemous, and some would say that Cass should be punished for making them.
In terms of rational responses, one of the best-documented and least controversial allegations Dr. Cass makes is also the most incendiary. One of the most authoritative hadith, the Sahih al-Bukhari, states that Mohammed's third wife, Aisha, was six years old when they married, and nine when the marriage was consummated. Although there is some dispute about her exact age, it is uncontroversial to say that Mohammed had a child-bride, and there is a great deal of Islamic literature celebrating their relationship. Muslims might respond to the claim that Mohammed was a pedophile by saying that such marriages were normal at the time, and that there were also child-brides in Europe. They might also say that it is not for us to judge the actions of the Prophet.
The attached article by Myriam Francois-Cerrah addresses the idea that Mohammed was a pedophile in the manner outlined above. The other allegations may either be accepted but treated as unimportant (that he was a polygamist, for instance), or dismissed as matters of opinion.