Depression is nothing to be ashamed of.
I did not mean to imply that it was a subject of which to be ashamed, but rather to point out that if you peruse the CC, SDN, and Chronicle forums, in short order you will see that everyone has an excuse. If a mood disorder is an excuse for bad grades, then don't mention it. Don't mention anything that comes across as an excuse.
On the other hand, Bartleby brings up an excellent point, if that mood disorder is your motivation and if it is well-controlled (remember, your PI wants you to do research that advances his lab, he/she is not interested in your having distractions) then, yes, that could give the adcom & PI great insight into your passion.