You're right!
Sometimes it's difficult to judge if the thumbnail is good enough to attract attention, so I recommend checking the game's CTR.
For my game it was less than 1% initially, so I added a gif, a tagline, and reassembled the screenshots. The CTR improved to a 2%. I tried experimenting with new cover images, but it affected the CTR negatively so I reverted back. Performing simple experiments like these, and validating assumptions through qualitative analysis helps to gain more clicks.
However I feel getting a good rank in a game jam is not dependant on number of ratings. Game quality is important too.