Is "How good am I?" a reasonable question to ask a supervisor?
I would welcome such a question from a student I'd worked closely with, but I'd also like a bit of time to consider before answering. 

But even better, is to ask - also - what you can be doing in the short term to get even better. Make sure the advisor understands something about your long term goals so he/she can best assist. 

However, this implies that you have more than a transactional relationship with the advisor and each of you recognizes and respects the advice of one another. Some people are "brutally honest" and will say things you really don't like  to hear. Others are too "conflict adverse" and will tell you things that sound nice but aren't helpful. You will need to judge for yourself the quality of the advice. 

But that is the reason for asking how to improve. It helps you directly, avoids personality issues to some extent, and helps you better judge the "raw" assessment. 

In any case, if you are good enough to ask the question, you are probably pretty good anyway, even without knowing much more. I had professors in my background of whom I would gladly ask the question, others of whom I certainly would not, and still others for whom I'd think long and deeply before asking. But that was more personality than anything. If trust is there, it should be fine. 

My own background somewhat influences my response here. When I was in secondary school I was told, on the basis of some standardized test, that I didn't have very good prospects and shouldn't set my goals too high or I'd be disappointed. This was largely based on mathematics. It wasn't a request to an advisor for guidance, however. The episode made me so angry that I decided to prove them wrong and later earned a doctorate (among other degrees) in math.