Mid-calf socks sit in the middle of your calf, roughly 8 to 11 inches from the heel. They're tempting because they feel less bulky, but they create a real problem for skiing: your ski boot shell will likely ride above the sock's cuff and rub directly against your bare skin. That contact leads to chafing and discomfort that can cut your day short.
There are a couple of scenarios where mid-calf can work fine. If you're spring skiing on warm days and your boots fit loosely, or if you're wearing them for après-ski and casual winter activities rather than actual skiing, the shorter height isn't a dealbreaker. Some skiers also prefer them for short, mellow sessions.
For most resort skiing, though, over-the-calf is the way to go. It fully covers the area where the boot cuff meets your leg, stays up without sagging, and gives you better warmth and compression. Think of mid-calf as a niche choice, not your everyday ski sock.