Granbull S/Cr - Decent Mewtwo counter?

It's not that your wrong in your thinking, it is just that there is so much better.

Gary > Granbull in every way shape and form.
Dragonite > Granbull too. It's stats being enough better and its moves enough (Crunch is a bad move in a general sense) that even with 40% SE vs 20% Stab dragonite wins.

Many older players likely have enough dust invested in Gary and Dragonite to round out thier teams.
and with dust currently rare, I wouldn't spend it on Granbull.

That said if I had a level 30 granbull, and if I was going for the 6 faint fast, then go back with new 6 approach, it would be on list for the 6 faint fast.

I thought in the same way, but later I found out that she is so fragile that she might be a Gengar which outputs less damage due to its defense stat.