A good Penzance day often includes St Michael's Mount, but the best plan is to treat it as part of Mount's Bay rather than a standalone dash. That gives you time for the walk, the views and the practical bits around timing.
Start by checking the tide and opening hours, then build the rest of the day around the Mount, Marazion and the journey back through Penzance. That way you can choose whether the day becomes a long coastal stroll or a more focused visit.
The key is to leave enough room for the return journey and a relaxed finish in town. If you try to stack too much around the Mount, you lose the simple pleasure of being by the sea.