Yes he is. His mother is a US citizen, so therefore he to is a US citizen. It's rather ironic that he supports amending the 14th amendment to do away with birthright citizenship, which would render him a Canuck for life.
Well, no, in Cruz's case the 14th Amenfment doesn't apply because he was born in Canada. The 14th Amendment only grants birthright citizenship to those born in the United States. But while the 14th Amendment guarantees birthright citizenship to those people and removes from congress the power to deny them citizenship, it is by no means the exclusive means of obtaining birthright citizenship. Cruz obtained birthright citizenship under federal law because his mother was a U.S. citizen and met the statutory age and residency requirements at the time Cruz was born.