Some rabbits may like to eat pumpkin. In fact most will probably nibble on it for a taste test. However, if your rabbit likes pumpkin you should know that, as with most veggies, rabbits cannot eat too much without getting diarrhea. This is because rabbits are not adapted to eating foods rich in carbohydrates (sugar and starch) which are very easily fermented by bacteria in their digestive system. Sugar and starch are like candy for bad bacteria which will then out grow the good bacteria (needed to produce vitamins and other nutrients for your rabbit) and instead will cause a shift in the pH and eventually gas and diarrhea. Ironically most rabbit pellets have too much grain (starch) and even use soy! These things cause diarrhea in the same way and is the primary reason why pellets have a bad reputation. See the graph below to compare.
Baby rabbits are better adapted to eating a high fat diet (like mamma's milk) that is also high in fiber. They need the extra energy and protein to support healthy growth. Most brands of rabbit food add grain as the source of energy but this is the very cause of diarrhea. Instead baby rabbits are far healthier when fed a diet that uses whole oil seeds instead of grain as a source of energy.