Yes, but the operative question here isn't whether it's mental illness, it's whether it's beneficial. Similarity to harmful mental illnesses is a reason to be really careful (having a very low prior probability of anything that fits the "mental illness" category being a good thing), but it's not a knockdown argument.
If we accept psychology's rule that a mental trait is only an illness if it interferes with your life (meaning moderate to large negative effect on a person's life, as I understand it), then something being a mental illness is a knockdown argument that it is not beneficial. But in that case, you have to prove that the thing has a negative affect on the person's life before you can know that is a mental illness. (See also http://lesswrong.com/lw/nf/the_parable_of_hemlock/.)
This is the argument to adopt a religion even though you know it's epistemically irrational.
Thus spake Eliezer:
It seems that many here might have outlandish ideas for ways of improving our lives. For instance, a recent post advocated installing really bright lights as a way to boost alertness and productivity. We should not adopt such hacks into our dogma until we're pretty sure they work; however, one way of knowing whether a crazy idea works is to try implementing it, and you may have more ideas than you're planning to implement.
So: please post all such lifehack ideas! Even if you haven't tried them, even if they seem unlikely to work. Post them separately, unless some other way would be more appropriate. If you've tried some idea and it hasn't worked, it would be useful to post that too.