Yeah I struggled with this question and did some research online and irl about it (including counselors, my psychiatrist..) General concensus seemed to be that you should feel free to mention it if it naturally comes out (explanation of bad grades is important after all) but you should not make your mental illness a major/main point or 'push' it too much to the point where it begins to seem more like an excuse or boast rather than a hardship. However, each situation varies so much, it is really hard to draw a specific line. For my college essay, I included my problems but only in very vague terms. It's also helpful if you try writing it out and then show it to people, asking if it works, because sometimes you can't tell if it will work or not until you read it. Hope I helped and good luck!