Yes, it’s a good idea to buy a removable slipcover for your futon. The cover does two things for you: One, it protects the mattress from getting soiled; Two, it makes your futon look nicer. When your futon looks nice, you’ll enjoy it more, just like the other furniture in your home.
Before you buy the cover, feel it with your hand. If it’s lightweight, like a bed sheet, it won’t hold up. It will fade and rip in no time at all.
Is the cover washable? Most of the better home furnishings fabrics are not washable, so ask about the cleaning recommendations. But don’t get carried away. Unless you have small children or pets, you probably won’t need to launder the cover often, if ever.