Twilight Highlands is probably not the best place to farm Elementium Ore, but, it isn't terrible either, especially if you have Herbalism to go along with your Mining profession.
Zillionaire has also put together a Twilight Jasmine Route if you are looking to combine Herb-finding and Ore-finding, or you can consult the map included with this article. The map has the Elementium Ore route and Twilight Jasmine route on it.
Of the several zones that you can farm Elementium Ore, Twilight Highlands probably ranks near the bottom for expected WoW gold profit when your ore has been placed on the Auction House. Twilight Highlands suffered from the same nerf that Deepholm suffered at the beginning of the Cataclysm expansion. Essentially, more than half of the nodes in the zone were removed.
When you have an ore or herb that can be farmed in multiple Cataclysm zones, it makes sense to pick the best and stick with it. Right now, Uldum is hands down the best zone to farm Elementium ore (and sometimes Pyrite ore). The only thing holding Uldum back is when there are too many other players running the mining route, which is when you can look at Twilight Highlands as a viable alternative. If you have both Herbalism and Mining, it is difficult to say whether Twilight Highlands or Uldum is your best choice. You should probably consult the auction house and see whether Whiptail or Twilight Jasmine is selling for significantly more than the other and pick Uldum or Twilight Highlands respectively.
Mount Hyjal Mining Route
Vashj'ir Mining Route
Deepholm Mining Route
Uldum Mining Route
Twilight Highlands Mining Route
Tol Barad Mining Route
Mount Hyjal Herbalism Route
Vashj'ir Azshara's Veil and Stormvine Route
Deepholm Heartblossom Route
Uldum Whiptail Route
Twilight Highlands Twilight Jasmine Route