As in a whole mountain that just so happens to be inaccessible.

Or is it just decoration?

Well iirc Skyrim came with heightmaps of the other provinces so Red Mountain should be there but inaccessible. I could be wrong though.

The Red Mountain is on the island of Vvardenfell, so it is not accessible from Solstheim. It's just viewable from Solstheim for the purposes of scenery.