No matter how dark or violent anything I write gets, it's still very tongue-in-cheek, I guess, is the best way to describe it. And that's not really something I've seen a whole lot of.
That's interesting. I'm not saying to change the way you write, but it's possible you are right that this might be holding your work back from being accepted.
I say this because I think I might have had a similar issue in my own writing. My writing teacher at the time pointed it out to me. She said one day I was going to have to move on. After a while, she realised that was my natural writing style.
Thing is, I was writing contemporary stuff for her (her skills were in the literary arena). When I wrote contemporary, I'd always be tongue in cheek. She said it created "ironic distance".
It took me a while to understand what she meant. It's hard to describe, but I'll try. Essentially, the way I wrote, even though I was trying (and thought I was) writing from a close, personal POV, my use of language and juxtaposition of detail created a situation where the reader saw an ironic view of the character that the character didn't themselves.
My cynical worldview infected my stories even though my characters weren't cynical themselves. Maybe if they had been cynical, it would have worked. In fact, when I wrote one that did have a cynical MC, in first person, that one sold. The cynicism came from him, part of his personality, rather than being a tongue-in-cheek narrator.
In fantasy and science fiction, that kind of tongue in cheek can come across as not taking the genre seriously, I think. A lot of people don't like that, and maybe editors won't buy it for that reason. When I write fantasy and SF, I don't suffer from ironic distance. Now, I can see the change in tone between my secondary or paranormal world stories and my contemporary settings.
I know how to get rid of it in my writing now too. But it's not something a critiquer ever pulled me up on. I think the stories work better now, at pulling a reader in, too. I had to consciously push past the tongue in cheek, and at first, I didn't know how.
This might not be your issue, but since you brought it up I thought I'd add my 2 cents.
If you are writing a farce (which some of my contemporary stories were, though I didn't know it at the time), or humour, then that tongue in cheek can be your friend! I think it can work in a story - but it takes more skill than I had at that time.
EDIT: Markets that might accept that kind of thing? Probably the ones that aren't always serious.
(Obviously.) My first, very tongue-in-cheek effort got to the final round at ASIM, for example.
EDIT 2: The trend towards dark: yes, I think it's very apparent. Latest 'best of Australian SF & Fantasy' I read should have mentioned in the title that it was mostly horror. Very dark indeed. Often bleak. Sometimes its because they set out to write horror, other times I think it's for that perception of 'reality' that others have mentioned. Totally happy endings seem less believable, unless it's a comedy or romance. But as a reader, I like a happy ending (usually there has to have been some kind of cost to get there though), and I'll keep writing both kinds. I don't like totally tragic endings either. They annoy me; I feel like I've wasted my time reading about the person's struggle if nothing at all is gained. Unless it's signalled that it's set out to be horror, I feel like the story is pointless if it ends that way. But that's just personal taste.