That's true Hydrolox is by far preferred for upper stages, particularly for interplanetary missions. However Heavy is so powerful it compensates quite well for it's compromises in that area. It can still boost double the payload to Trans-Mars Injection than Delta IV Heavy and more again than Atlas(1).
(1) As Dom Toretto always says, American Muscle beats Import every time ;)
They can do it of course, Falcon Heavy could launch a car in the general direction of Mars. But they are limited by their use of kerosene in their 2nd stage and its low specific impulse compared to hydrogen.
BFR and it's Raptor engines are better suited for that purpose. But we don't have a BFR yet.