What is the correct term for paper that has become wet and deformed as it dried out?
Solution 1:
Cockled. That is the paper conservator's technical term for wavy or rippled paper. See e.g. Laura Schell paper conservator:
Cockling Warped and wavy (cockled) paper may be restored to a planar condition by controlled humidification and flattening.
Solution 2:
As mentioned in some comments,
Warped - is the more general laymen term, but not quite as specific as Cockled (which I had not heard before)
Become or cause to become bent or twisted out of shape, typically as a result of the effects of heat or dampness[1]