I know that feeling, and usually my (perceived) incompetence vanishes at least after day three of riding. I have done a lot of bikepark riding this season, and usually I feel not too good on my bike the first few runs. About two weeks ago I did a few one footers on tables at my local bikepark, last week I was in Austria and was happy to clear the tables on my second day, not even thinking about taking a foot off, and today at my favorite jump spot I felt like a wet sack of cement.
I think the ups and downs of applying the skill set necessary for riding mountain bikes is totally normal, and nothing to worry about. There are too many factors part of the whole thing, a bad night's sleep, some trouble at work or home, ot enough recuperation, who knows?
The big part for me this year was to realize that in the end even the rides where nothing seems to work out are good ones in the end, if I ride shitty that does not longer mean that I can not enjoy myself or the scenery or the company. There is no guarantee in life, so maybe tomorrow is a better day. I can ride for fun and do not have to ride for a paycheck.