I don't recall Cruz saying anything about anyones military service, that being said I am pretty sure McLame left a goodly part of his brain in Vietnam, not sure that I could have stood up to what he did, but that doesn't mean he is fulling his duty to his constituents now.
With a class rank at Annapolis of 894 out of 899, it's a good bet he didn't show up in Viet Nam with a lot of brain to start with.