It can be serious, but if caught in time, its mostly cosmetic. Along with the pyramiding come metabolic bone disease. And the shell material is full of holes, rather than more dense material. Look for another thread and I posted some pictures of a pyramided carapace cut in half so you can see the porosity.
Not on its own. It would have to be coupled with MBD. Just from my experience with rescued Sulcatas. I've seen some pyramided sulcatas that are larger than what would be "normal" for that age, by the same token, really badly pyramided sulcatas, that also include MBD, are usually smaller than normal.
Here's an example of pyramiding and MBD. Their internal organs are squashed and they get too weak to use their legs. While I didn't see Toby first hand I was told it took months after he was rescued before he could raise his plastron off the ground. MBD is also painful to the tortoise, possibly like arthritis or fibromyalgia is to us.
I have one that look slike Toby. He is smaller and has not grown much at all, but he has grown some. Compared to the others I have, its almost nothing. But he loves his mazuri and greens and cruises all over.