One can only hope.
There's one mountain in Cuba that's 6,500' high, but that's the exception. I think we're far better off hoping that Ernesto rakes Cuba for several hundred miles than expecting the mountains to doom the storm. Although, based on the latest recon report, it looks pretty injured at the moment.
The mountains are located along the southeast coast near Gitmo. And once you cross them, it's pretty much flat all the way to Colorado.