Mayonnaise is made with raw egg yolks and olive oil. It will loose its texture (will become watery and transparent) and loose the taste when microwaved. Otherwise,It is safe to eat but do not wait too long before eating it because it will grow bacteria.
Another fact about mayonnaise is since it contains raw eggs, you can be contaminated with Salmonella bacteria and as far as I know, it may cause miscarriages on pregnant women.