I feel like on Itch, more people choose cover images that showcase their game graphics
You might be able to test your hypothesis on these and similar lists' top pages.
https://itch.io/games/top-rated/tag-deck-building
https://itch.io/games/top-sellers/tag-deck-building
Most devs here do not have a budget, so no professional cover art. The cheapest way to have such an image is to take a screenshot from the game. So, while I do agree that gameplay pic is often used, I do not feel that this is done, because people think it increases success, but out of necessity.
Of course there could be a feedback loop, if regular users would expect their favorite games to do similar. Hence my suggestion to see if your observation holds true for your target audience.
Personally, I am on the fence about this. I would react to certain key elements. Seeing that this is a card game at first glance would peek my interest more than seeing that it is a horror game. But if I am interested in card games, I probably already would use a tag like deck building which would nullify that benefit.
But I think this is the high art of marketing psychology ;-)