There are no restaurants on the summit of Sugar Loaf mountain (although there is a snack bar that sells savories, sandwiches, drinks [Caiprinhas, local beers], etc.). However, if you want to have a meal, you can eat lunch at Abençoado Bar at the food court at the top of Urca Hill, the hill half way to Sugar Loaf. The bar/restaurant serves lunch (typical tasty Brazilian food) from 12 noon to 4 p.m., but no dinner.
Another great option is Urca Bar. Although not a bar only, neither located at Sugar Loaf, is one of the most popular options among locals living or visiting the Urca district, where Sugar Loaf Mountain is located. To get there you either take a 5-minute taxi, or walk for about 20 minutes. Beware that the place is very busy at lunch times.
If you want a complete updated list of restaurants nearby Sugar Loaf in Urca district (and in other neighborhoods and areas such as Copacabana, Ipanema, Leblon, Leme and Santa Teresa), get a copy of My Rio Travel Guide: The Essential How-to Visitor’s Guide to Rio de Janeiro.