For the photographer, Imposter Syndrome is worse. Not believing in yourself can have a terrible effect on your work. You're better off thinking you're better than you are because sometimes you'll accidentally succeed even when failure was much more likely. Plus, in today's world, there's very little consequence for failing. It's pretty easy to control your image online, assuming you don't go viral in a negative way, and word of mouth is a lot better tool for promotion than negative persuasion. On top of all of that, price and branding are probably your biggest assets as a photographer. The average Joe doesn't know the difference between a good photo and a bad one, but they sure know the difference between a cheap photo and an expensive one. And if your website and business card/office say "professional", they'll probably be inclined to agree.
For the client, Dunning-Kruger is worse. Clients are always happier if you under promise and over deliver. Sure, there's something to be said about talking up a bad product to the client to get them to like it. But I'm sure they'll eventually compare your photos to other ones like it and eventually decide on their own if it was worth the money or not.
In any case, greatness doesn't come from comparing yourself to others. It comes from constantly striving to do better than you've done before. So it's probably best to not worry about how good or bad you are, but rather just focus on how you can improve. Life's a journey, not a destination. So treating it like a race against others won't help you because there's no finish line to cross. The goal is to each day get a little further down your path.