I'm the last guy to go PC, but that remark is just so wrong on so many levels.
1. Why would that make it OK?
2. Why would Trump even being "doing" a mentally disabled person when the reporter is physically disabled?
3. While the term "retarded" is not entirely off the table, the term "retard" certainly is. It's not quite as bad as the n-word, but it's right up there with the c-word.