Not IMO.
I think Thurston is clearly a very good player.
In his time at the cowboys he has only led the Cowboys to 2 top 4 finishes in 11 seasons (one less than us) and over that period have only won about 6 finals (again less than us). The cowboys have had enough good players that if Thurston was an immortal he'd have done better than that over the period.
Thurston is the king of the big play, however is consistently in the top few for errors and missed tackles every season, Joey and Wally were above Thurston in this department, both amazing defenders and made much less errors.
The Queenslanders have been dominant in his time in SOO, and Thurston, clearly a huge part of that. It must be said though, NSW have also been at their weakest over the same period. Australia from the late 60's has rarely lost test matches, until the last 7 or 8 years, where the Kiwis have won a few big ones against us, and most recently have had the wood on us. I think Australian footy in general is a little below its peak at the moment, (mainly cause NSW has gone through a bit of a doldrums). However this last paragraph is only very secondary to my main point and that is in regards to the Cowboys lack of top level success and Thurstons errors and defence being a factor in that. (only compared to the very best)