The Brexit will easily overshadow any other "achievements" he may have made as PM and the fact that he's resigning over it pretty much validates that. If the Brexit turns out to be a complete disaster (which I think it will be*), then history will not look very fondly at him.
My knowledge of the history of GB politics is too narrow for me to judge whether he will be the worst Prime Minister the country's ever had, but to me it seems that very well could happen.
*I've had a major shift of opinion on this recently and the short-term economic and political ramifications have me extremely anxious now.
Given how he just presided over something that may very possibly led to end of the "union of Kingdoms" which goes back all the way to 1707, I honestly don't know how any other PM can possibly top that.
Well, he'll be in good company.