Hitting the ball down has nothing to do with bat path.
Hitting down means making contact with the top hemisphere of the ball.
Hitting up means making contact with the bottom hemisphere of the ball.
Line drives are centered contact.
You will transfer the most power into a line drive because more of your energy is converted to speed.
When you hit up or down, some of your energy is converted to spin. (This is actually a good thing.)
For grounders and dingers, spin is good. For a ground ball, a little top spin makes it harder to field the ball because it “hops.”
For a dinger, a little backspin actually helps the ball float and stay in the air longer, letting it fly farther.