As a comic creator on Tapas, I think the cover is one of the most important promotional assets, moreso for comics than novels, because when you look at the listings for comics, we don't get the blurb displayed alongside, or even the title, it's literally just cover, genre and how many likes.
For a comic, because your title won't be written in text, the title on the cover has to be comfortably readable at the size it'll be on a phone screen (about the size of a postage stamp), and the artwork has to be so eyecatching that based on just that artwork alone, with no other information, somebody will click or tap your comic.
Also, covers are what will be shown if the staff feature your work. Tapas don't want to put things that look amateurish on their front page, so one of the best things you can do to raise your odds of a feature is to make a cover that looks really neat and finished with nice typography, like it belongs there.
Thumbnails are less important than covers on current Tapas. You used to see them used for features, but now their main purpose is on update notifications. Keep if clear and very easy to tell it's to do with your comic and you should be fine. Episode thumbnails should ideally be unique to each episode; it just makes things look a bit more finished and like you put in that bit of extra effort.
The only other thing to say about thumbnails is that they are what's used when your comic gets recommended under other comics either by being on somebody's reading list or just by Tapas' algorithm or something. So at higher sub numbers (1000+) or possibly likes (10k+) you might find an eyecatching thumbnail pays off for getting subs off those recommendations.
