People want to know what they are going to get when they purchase a book, and spend their time reading it.
By telling people the genre of the novel, the ones who want to read that genre will purchase it, and those who don’t want to read that genre will simply pass by it without a second glance. People don’t want to pick up a book expecting a murder mystery only to find it’s a cringe-worthy cliché romance between the motorcycle-riding, leather-jacket-wearing bad boy/gang leader, and the awkward, shy girl who does nothing but read (even though she doesn’t pick up a book after the first chapter has ended).
People expect the book they pay for. If they pay for a murder mystery, the want a murder mystery. If they pay for a demonic clown that eats children, they want a demonic clown that eats children. Don’t give them the opposite of what they’re looking for.
If you’ve written a romance novel, give it a lovey-dovey title and make the cover as romantic as possible. If you’ve written a murder mystery, give it a suspenseful title with a dark and mysterious cover. Don’t give the novel about a demon terrorizing a town a cover with two people staring lovingly into each other’s eyes, and a title that screams “IF YOU LIKE STAR WARS, READ THIS!”. Readers will be annoyed that what they thought they were buying is something entirely different.
People want what they payed for.
People want what was advertised.
People want what you promised them.
People do actually judge books by their covers.
Give them what they want.