To be fair, Cruz hasn't accomplished much of anything in the Senate as he is one of the few attempting to be responsible. I don't see any reason to believe, at this point, that he would be any more successful as the President.
If he is incapable of building a consensus in the Senate, why should we be confident that he could do any better in a higher office?
Probably the only person more despised by Congress than Cruz would be Trump. So if Cruz would not be able to get anything done, why would anyone believe Trump would do any better?