Is a non horizontal horizon a problem?

I'd say fine too.

Thanks for posting the image for me, having a little trouble there.

I was looking at the image and wasn't sure if I had the camera off or not. I looked at the fence, then the trees and of course the horizon was on a slope and then I thought what the hell it looked ok to me!
So I thought I would seek a second opinion.

The central cloud;'s left edge looks like it's burnt out which is a bummer as otherwise the clouds look good. That said a really wide angle lens (sub 15mm) would have made those clouds fly by which is often a nice effect.

Seems ok to me, thing is most horizons of this nature are rarely level anyway. In fact, unless you are looking at a large expanse of water it is unusual to get a visually level horizon.

Davex.

There are times when a sloping horizon, or the illusion of a sloping horizon are OK. In a wide angle close up graphic image for instance its sometimes acceptable to tilt the camera for dramatic effect.
The illusion of a sloping horizon can be given in a picture like yours where the hillside slopes; or when looking across a lake at the opposite shore which runs diagonally to the plane of view.
I agree with RR that your picture seems level but the main cloud is a bit burned out.
david