A relaxed fit base layer might feel more comfortable at first, but it's not a great choice for active days on the mountain. Because it drapes loosely rather than hugging your body, it creates air gaps between the fabric and your skin. That means sweat can't wick away efficiently, and those air pockets can turn clammy and cold—especially when you're sitting on a lift after working up a sweat.
That said, relaxed fit does have its place. It's perfect for après-ski lounging, cabin wear, or very casual resort days where you're barely working hard. If you really can't stand a slim fit, a regular fit is a better compromise—it still gives you some breathing room without completely sacrificing moisture management.
For most skiers and snowboarders, though, a slim or form-fitting base layer is the sweet spot for staying dry and comfortable all day.