It would not be unknown when removing wallpaper over drywall, especially if it was not properly sized, to take some of the covering paper off of said wallboard. What is not normal is for the gypsum underneath that paper to be crumbling. If it is in fact minor you can skim the wall with drwyall compound (taping or topping type) and sand it down smooth. Might need to do it a couple of times to get it right. If worse, you might try some of the quick set compound but smooth it well because it is hard to sand. This might stick the loose material back together. If really bad you need to investigate why it is so bad and fix that and will need to either tear it out and start over OR go over it with 1/4 inch wall board to cover it up. You can either just patch your round corners (that I suspect are really plaster) and board up to them. 1/4 inch might bend enough to cover them, depending on the radius of the curve. Wet the back and let it sag over a saw horse or such into a curve. You could make them square if replacing what is there. That would take a bit of framing to accomplish. Good luck.