The problem with clouds though is that they are really abstract, in the sense that most of the clouds we see, are either further away than we think, or closer. This is why most artist goes for something typical, for that makes it easier for the viewer to accept.
That is where it becomes difficult when creating imaginary clouds, and a cartoony, stylized one at that. My best advice would be to look at some old fantasy work that fits that style and try to bring the fantastical sense into life.
As for the grey, it does look like a black spot on the canvas that ruins it, but at the same time, it isn't necessarily wrong. In this case, this grey area would be a bunch of clouds that would cover all of the landscape behind the dragons. I think the main way to go about fixing these clouds are artistic decisions, which will be up to the OP. You could try to soften the dragons, making them more cloud like than actual hard surfaced dragons (unsure whether they're real dragons or literally clouds). Another thing that seems weird, is atmospheric perspective like Moirae pointed out, especially with the dragon head in the middle.
Keep in mind that you are not painting a small section of a table or something, where pretty it is pretty much one light with some texture. These clouds cover several miles and have different hues and saturation to them. There is no bright sun in the painting, but there is a very strong blue sky, try to reflect that onto the clouds. And depending on the kind of look you're going for, subsurface scattering can be applied.
Anyway, I think it looks illustrative and cartoonish as it already stands, and I totally accept the way they match together. But, this can definitely be changed, and to do so, I think you might have to do something about the look that you're going with for the dragons. Anyhow, this is some really nice work. Hopefully this was helpful! 
