If you are using all cotton, the worst it would do is scorch it.
Polyester will melt under high enough temps.
And with polyester, it depends on if it’s 100% or a cotton.poly blend.
When I iron some of my quilting fabrics, I often really nuke it.
The same with my husbands hankies and our pillow cases.
You just have to try it on a scrap piece of fabric to see how high you can go, at least with the ones you make yourself.
Might not a hair dryer work just as well without scorching? On most any mask material?