Perhaps in that case you could reflect the light heartedness and the darker undertones in the cover. Because right now the cover just looks bleak so when you read the light heartedness right at the start there might be some dissonance. The initial impact of the story matters too so by making the first impression to be light hearted but yet something feels a little off in the cover might lead to setting the mood you want without letting the sudden dark turn seem out of nowhere. You should allude to these things before they happen, dramatic changes need a bit of set up.
I actually have an example of something like this, because I was drafting a story similar in tone to yours. Basically beautiful exterior but dark interior.
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So the idea for this cover was to seem off at first glance. The Psychedelics title text that almost hurts the eyes and the crimson blood contrasting with the cheerful greens, yellows and blues. Even the title reflects it subconsciously, which is that Dreams can be good and bad but friends are usually good. So the blood is under the dreams. Most people won't pick it up but by alluding to things, the mind can pick up things subconsciously. It won't work for all your readers but it will work for some.
PS. I may check out your book later, but I don't have the time now.
