I fly over the Himalayas a lot in the sim. The quality of the mountains vary, obviously its governed by the quality of the mapping data available to the Bing maps team. Generally though they look pretty good, especially from a distance, like in these photos.
I think what lets the look of mountains down generally in the sim is that the satellite photo of any given mountain area gets stretched over the terrain mesh. It can look ok but it can sometimes turn into a bit of a blurred mess.