Yeah he seems to think he's a bigger fish than he really is, none of the others on that council really seemed to like him.Gideon seems like the kind of person that has to fight for every inch of power with his scheming so he's probably upset at Thrawn seemingly being ceded authority without earning it in his eyes.
As his Mando cosplay kind of indicates, he also doesn't seem to have a cultural identity of his own so maybe he's also bitter Thrawn is a Chiss and he can't be cool like that.
He definitely desperately wants to be something more than what he ever will be.