it doesn't matter if it's a 'bad day' for you on the assessment day. you have to answer all the questions, on the form and in person, as if it is a bad day, the worst day for you, because that's when you need the blue badge.
most blue badge holders have good days and bad days, but it's the worst day that counts, the days when you're in pain, can't walk, etc.
don't feel guilty (which is what a lot of applicants do) because when you're having a bad day, the badge could make such a difference to your life.