I mean.. nobody (no creator anyway) is going to sign up for a site dedicated to promoting their work as "underappreciated". I don't think you're going to find a single specific site dedicated to the kind of categorization you're looking for. I suspect you're going to have to get good at trawling through many, many different sites and filtering for the things you're looking for.
That said, there are plenty of interesting and underappreciated projects on Github, Sourceforge, Savannah, Gitlab, Bitbucket, etc. as well as sub-reddits like /r/somebodymakethis, /r/somethingimade, etc., Kickstarter, Tindie, and so on. You might also get some mileage out of exploring social media sites that are less mainstream, like various Mastodon sited or other parts of the fediverse. You'll probably find some iconoclast types there who don't share their work on the "major" sites.
