I say yes, because while Serra was a bigger upset it was the 'punchers chance' kind of upset.
TJ nearly destroyed him in Rd 1, went on to dominated 3 more rounds clearly, and then finished him in Round 5.
The performance is the true awesome thing about this. Serra beating GSP showed that anyone has a punchers chance, but GSP proved in the rematch what a fluke it was.
This was no fluke, it was domination from beginning to end, from a huge underdog. That's why I say it's the biggest upset ever.