Source: am a furniture maker, professionally. I've switched from using a joiner's mallet to a lump hammer, so I have experience with both. The lump hammer faces are perpendicular to the handle, not angled in the slightest. Better lump hammers have slightly domed faces, so if you're walloping a piece together and don't have time to grab a block to protect it (time moves faster once the glue is on), you're less likely to leave a bruise from the edge of the face if you don't hit it with the face of the hammer/mallet dead square to the wood. Joiner's mallets generally have flat faces because it's easier to build them that way.
In practice, you also aren't usually swinging in an arc when you hit a nail or chisel either. You start your swing in an arc, and transition to a flatter trajectory as you approach the end of the swing so you have less motion in the plane of the face of the head as you're hitting the nail/chisel. Or in other words, you want the head traveling along the axis you're trying to drive the nail/chisel so you don't skew it off axis.