Thanks for expanding. I've seen him in a few of the 90-some titles he's been in. Favorite is the quirky She Came to Me. I think he can usually be counted on for solid performances and is especially good at delivering wry lines with the right amount of deadpan expression. I also admire his insistence on being cast as an actor, not as a little person. Most of the roles I've seen him in (discounting GoT) he is just a a person and few references are made to his stature. His also garnered some critical praise for titles like The Station Agent (which I've yet to see), Three Billboards Outside Ebbings, Missouri (which I like a lot) and Cyrano (not so much). With more exposure he might grow on you. (Pun intended.)
Your response is suggestive that you might be a tad biased in your own assessment. Our admiration of the actor, our appreciation of his work in other roles, his insistence on his own dignity, is irrelevant to the question of "Best Acting Performance in Game of Thrones." I think this sort of bias explains why he is a overrated in this role.