Lorenzo, I will assume your mountain bike has knobby tires. Forr many years my only bike was a mountain bike, and I began to commute with it. When I changed the tires from knobby to smooth road tires, the bike became very nice on the road. If cost is a consideration, you could buy a 2nd set of wheels and put street tires on them, so you could switch them out as desired. And for even less money, but more work, you could just buy smooth tires and mount them on your bike when desired.