There was someone in the comments (either here or on MD) who figured out that Mask had an undercut several chapters back. I went back and looked at all of Mask's previous appearances, and it's really hard to tell. In most chapters it just looks like darker shading around her ears, but in the ones with Onibi's birthday and the one where Vampy pushes her down (especially the color shot) you can make it out better.
I really want to see her with it combed over or styled up now. It's kinda hot.