Great thread!
I've done odd bits of design work, but not to the extent of a graphic designer, but I have a decent sub count and have been featured a number of times, so I'll focus more on the "what works on Tapas specifically" side of things.
1: FACES, FACES, FACES.
Tapas covers, like Youtube thumbnails, practically always perform better if they prominently feature at least one face that's emotionally engaging the audience. Even on novels (not necessarily the case in print, or even on places like Radish). Characters are a big selling point on the platform, so covers are often a bit more like East Asian Light Novels than current trends in print covers, usually strongly featuring character designs rather than leaving characters vague and focusing more on vibe or design.
2: It needs to look good at postage stamp size.
Don't be subtle. Go bold, high contrast, big text, really clear, because most Tapas readers will not see your cover at book cover size; they'll see it in the listings or a feature on their phone or tablet displayed at roughly the size of a postage stamp.
3: Eyecatching and obvious beats clever.
If you have a clever cover design (like an optical illusion, cool use of negative space or interesting detail), it needs to be eyecatching first so people don't just scroll past it. Tapas users tend to like to know exactly what they're getting from a series, so misleading or subtle covers and blurbs or ones that hint mysteriously will tend to underperform compared to ones that literally just spell out the whole premise of the story with all the subtlety of a sledgehammer. ie. If Twilight was on Tapas, that famous iconic cover with the hands and apple (which is a brilliant cover, I'm not even a Twilight fan at all but damn, that's a great cover) probably wouldn't perform as well as an illustrated cover showing Bella looking adoringly at Edward looking cool and aloof in the moonlight all drawn in a manga/manhwa esque style to make it very clear it's a story about vampire romance.
4: Leave off all unnecessary text.
Small text won't be visible and the last thing you want is your title getting overshadowed by your name when concepts and characters tend to sell comics over famous names on Tapas (unless you're as famous as Sarah Anderson). The only necessary text is your title, which should be bold and easy to read. Sadly yes, this means that really cool typography-based covers that might do great in print may not work well here.
5: Cheesy beats tasteful.
Nobody is reading your comic on the train with the cover visible, and if people are really bothered, they can even hide their library, so you don't need to make your cover pretend to not be delicious cheesy trash for the sake of people's dignity. Tasteful, restrained and muted covers will tend to lose out to "sexy anime guy with a flaming sword" or cute people blushing at each other surrounded by cherry blossoms. Embrace the cheese.