Maybe. Mary Sue-ism and self-insertion are often intermixed, and I might be conflating them. Still, there’s a difference between characters reflecting facets of the author’s personality and a character who the author explicitly claims is his avatar in the world. Mind, I don’t begrudge Greenwood this, as I firmly believe the creator of a world can do what he damn well pleases with it. It’s just a very blatant example of self-insertion and self-idealization.
IIRC, I read an interview where Greenwood said that Elminster is not based on him or supposed to be him.
That so? I could have sworn I’d read as much in an afterword he wrote in one of his books. If that’s the case, I retract that argument entirely. 
Not to cast undue doubt, but that’s also exactly what a Mary Sue author might say if you asked them about it, to their face.
“He’s completely NOT based on me! He has BLONDE hair, and both of my parents are still alive!” 