A moderate amount of corn is fine and many rabbit feeds (including the one I've used for over 15 years, contains corn). Corn is a good source of carbohydrates for rabbits, but too much can cause serious GI upset. Good for production rabbits because they need higher energy feeds - bad for pet rabbits.
Corn is approx. 9% protein, so could increase the rabbit's protein level too much if used in addition to commercial rabbit pellets. On the other hand, corn can be a good source of protein if the rabbit is on a limited pellet and limited alfalfa diet.
In some areas, corn is the staple dry diet for rabbits.
Because of the lower energy requirements of a pet rabbit, I would not make corn a regular part of their diet. Pet rabbits fed on a diet high in corn can suffer from obesity, leading to health problems. Corn can be used as an additional energy source for rabbits housed outdoors in very frigid weather.
Pam