Is a GoPro or smartphone camera good enough for ski videos?

It's true that it really depends on what you want to do with your video. A GoPro will give you better stabilization and better durability on the slopes, especially if you mount it on your helmet or chest. With a smartphone, you're a bit limited because you have to hold it or mount it in an unstable way, and even with software stabilization, you're going to see grain as soon as there's low light (like in the shaded areas on the mountain).

That said, honestly, if you just want to film a few runs to share with your friends, a decent smartphone will do the job. It's really more about the mounting - find a good rig to fix it on you rather than holding it in your hand. Like, even my video from the last couple weeks where I filmed some stuff on my phone and then compared it to footage from a borrowed GoPro, the difference was clearly visible once we put them side by side. But it wasn't a disaster with the phone either, just a bit choppier in tight turns.

If you're planning to do this regularly and you want pro-quality footage, then yeah, investing in a GoPro makes sense. Otherwise, stick with your phone and mount it properly - it can already work pretty well.