A UPF 30 rating blocks about 97% of UV radiation, which is solid protection for most spring skiing days at typical resort elevations. If you're skiing primarily at moderate altitudes and aren't particularly fair-skinned, UPF 30 is generally plenty for a base layer—especially since most of it will be covered by your mid-layer and jacket anyway.
Where UPF 30 starts to fall short is at high altitudes (think above 8,000–10,000 feet) where UV radiation is significantly stronger, or on those intense bluebird spring days when the sun reflects hard off the snow. If you burn easily or you're doing multiday spring tours at elevation, stepping up to UPF 50+ is worth it.
The real takeaway: UPF 30 is a great sweet spot for general spring resort skiing. Just remember that your base layer only protects the skin it covers—don't skip sunscreen on your face, neck, and ears regardless of your UPF rating.