The other day I was writing in the morning, as is my custom. I had a cold and wasn't concentrating very well and I am sure I was only a shadow of my usually brilliant self.
Yet this "bad" writing session was still a good one. My manuscript grew by three or four hundred words. I generated some ideas I could develop later. Moreover, I maintained my continuity in working on the project.
One of the most amazing things I have learned from my own Stupid Motivational Tricks is that there is no such thing as a bad writing session. The writing does not care how you are feeling, it just needs to keep on getting written. The "bad" sessions just get averaged in with all the mediocre, so-so, pretty good, not bad, and kick-ass sessions, with the overall result being a wonderfully productive week or month or year of writing.
So if you don't feel so hot. If you feel uninspired and mediocre, that is an excellent time to not skip your writing. It might be even more important to write on those days than on the days when conditions are propitious.