Book covers are very much a case of the visual media speaking, telling us what we need to lure us in (so too album covers* back in the Ancient Years when I bought vinyl & then CDs). One of my favourite assignments as a university teacher was to have my freshman "read" advertisements. Where is the eye pulled? What do those colours convey? Are there models? If so, what do their postures, eyes, expressions, attire (etc) tell us? What is the tone? How does the font fit into the overall message? What about cover copy?
We "read" all of that from both our personal perspective/experience and from our current historical & cultural contexts. We do all of that very quickly.
So while we shouldn't judge a book's MERIT based on the cover, we are--because we are trained to do so--judging a lot of other things. The final decision/judgment is whether we will open the cover* to proceed to judge the contents.
I am thinking about covers extra because my new book just had a cover reveal in USA Today. (Go here to see it.) We went through well over 2 dozen covers to reach that one. It was a verrrrry long stressful process that involved me wringing my hands and (I suspect) my editor and agent pondering ways to get working voodoo dolls.
I can't go too much into the Carnival of Souls cover. I will say that the masks are a very important plot point (esp the white mask), and that the burning of the mask is significant. What I can do is take a look at the covers of the two books I read & recommended.