I personally feel masking is mostly about design.
Having said that it has it's place and wether that is in art or not is really a debatably subjective notion. I tend to look at design and art as a sliding scale. Design being about solving a problem (or illustrating a solution) and art is more about asking a question or sharing a human emotion. This isn't a hard-fast rule, only a starting point for me. Masking then, tends towards the design side of things.
Today I wanted to take it easy emotionally speaking. Wanting to go a little further with masking and design, I setup a couple cards with the intention of removing any free form expressive or emotional marks and just mask everything. So I did and the results are decently pleasing. Artistic? Not really. Still worth looking at the sliding scale of art and design though.