Speaking as a foodie who recently discovered his heritage is so Texan he can trace his lineage to The Old Three Hundred and whose family was given title to what is now Brazos State park by no less than Stephen F. Austin,
It's a bland-palated bastardization of norteno mexican food as interpreted by a bunch of midwesterners proud of their drawls.
Right - that part of the Rio Grande Valley where the white people showed up and got their asses kicked by the Mexicans, not that part of the Rio Grande Valley where the Spaniards set up house in the 1790s.
Authentic? Inauthentic? Don't care. Tex Mex is what happens to Mexican food when it's prepared for people who hate flavor.