Is it true that mascara causes damage to eyelashes?

If so, what kind of damage?

Good quality mascaras they are made by big cosmetic brands will never damage your lashes. Of course, a poor quality mascara can damage your lashes. They may cause falling down of some of your lashes or to make them thinner. To prevent any of those it really helps if you use a lash primer. And never ever go to bed without removing your mascara no matter how tired you are. That's very bad for your lashes. They renew overnight.