A fanwork genre is a terrible idea and would be asking for legal problems.
Should Fanworks genre even be added?
No.
If added, will it be barred from spotlights in the "Popular" and "Trending" tabs?
It shouldn't be added.
If added, will you stay {on Tapas}?
Until Tapas gets sued/DMCA'd out of existence, sure.
Of note, Tapas has noted that fanworks are not eligible for ad revenue/monetary gains.
Because Tapastic probably has at least 1 laywer and knows better. The fact they even allow fanworks to exist on the platform is basically playing a game of appealing the assumption that original intellectual property (IP) owners likely don't care enough about some 16 year old kid's fandoodle comic to actually bring suit for copyright infringement.
All fanart and fanfiction is technically copyright infringement. However, the companies that own large IPs usually know better than to pursue some kid's Tumblr blog. For one, it isn't great PR to sue or penalize your own fans for creating fanwork. Two, your average fanartist isn't interfering with their market shares or profit in a way that justifies the cost of pursuing it. A freelancer with like 2k followers selling anime pins is likely not making enough money to even cover the front end fees associated with a lawsuit. At most, a DMCA takedown is filed with the site owner. Major companies gain nothing by telling people they cannot produce fanwork. For the most part, it's free advertising and promotion.
However, they absolutely can and do have the legal right to make people stop producing fanwork or using their characters in any way. The more you're explicitly advertising the fact you're profiting from their IP the more likely they are going to come after you. Fanwork can exist as long as it isn't making too much money. The idea that you would actively draw attention to the fact you allow copyright infringement on your platform isn't the best idea. Most likely, nothing will happen. But in the event it does, having a "fanwork genre" is explicit acknowledgment that you are facilitating and encouraging copyright infringement.
References
Zambito v. Paramount Pictures Corp. 613 F. Supp. 1107 (E.D.N.Y. 1985)
Walt Disney Production v. Air Pirates, 581 F.2d 751.
Here are just a few examples of tests used in determining whether a work infringes on a copyrighted character. While fictional characters are not per se illegal in the Copyright Act, common law had established that you cannot used copyrighted and trademarked characters.
For clarification, infringement does not require financial gain. The mere act of creating a "copy" is enough. I'm not going to go into the definition and common law background for what constitutes a copy, but just know that all fanwork is technically infringement. The IP holders just have no incentive to pursue it usually.