If you knew an answer to that one, you could make lots of money in marketing and advertising.
In practical terms, just sort browse by popularity, as should be the initial sorting. You might see, or rather not see, a certain trend.
Well, after you noticed, how many horror games there are and filtered them out.
https://itch.io/games?exclude=tg.horror
The only trend I can make out, people put the title of the game in there and most do not look professional - compared to the thumbnails on Steam. As is to be expected from indie games.
Everyone looks for different things and has different tastes. Appealing to everyone only leads to disappointed players. So my best guess is, making a game cover that captures the essence or at least the art style of the game, so it might attract people that might like the game, would be a good idea. But I would not put too much weight on this. If it were very important, one might really see a trend among the popular games. While it is a weak conclusion, the absence of a trend suggests to me, that the cover has not that much impact on a game's popularity. Maybe if you filter for certain tags or tag combinations, some trends will emerge. My guess would be, that horror games aim for creepy/shocking covers...