Egghead Sanji was expected by some to be one shotted by Kizaru after doing nothing against him, expected to get hype tooled by S-Shark and expected to be useless against the Gorosei. Sanji fans never said he would defeat Kizaru in a 1v1 like some, they never said he would defeat a Gorosei in a 1v1 like some, nor neg diff characters that he obviously didn't, like some. They only believed that he'd be of use against the main enemies of the arc and show his progress, which he did, even preforming unprecedent feats that neither his fans nor most readers expected. So no, he wasn't overrated, because he preformed more than what his fans expected and not as bad as his haters wanted while surprising neutral fans who didn't anticipate much anyway. And when you compare him to the rest of the straw hats, with the exception of Luffy of course, he stands out as having been the most active/involved and overall most useful.