He didn't get fired at Ole Miss.
And I'm not totally sure, but IIRC, his last couple recruiting classes at TTU have been pretty highly rated. 20-17 record, 2 bowl games... That ain't all bad after the dumpster fire that TTU was after the Leach era.
It's also worth noting that Tuberville was 3-2 against Saban while NS was at LSU.
I don't know if Tuberville is great, but saying "he's always been a mediocre coach" isn't exactly correct either.