ABSOLUTELY, yes!!!! Much like handwriting! I normally have very nice penmanship, but there are days I cannot write at all....even writing out a check seems difficult.
And piano playing is no different. Now, granted, level of difficulty comes into play here...if I am accompanying vocals, no bad days...at least not discernible. But works of substance..most definitely.
Not to worry. But I've often wondered about concert pianists and their "bad days." Wouldn't it be awful to fly to a sold out venue and have an "off" day? Talk about pressure.
Like others have said, on those days when I cannot seem to get in step, I usually either play things that don't require much from me or hang it up all together and read a good book! haha