Felessan compares Elvhenan to Orlais. Not Tevinter. And even if it was like the Imperium, I don't think the Dalish clans know that it was that way. Part of the reason Felessan has contempt for them is that they have overly romantic notions of what their past was like.
The members of each clan are different. But as a whole neither of the whole clans we'd seen so far have been "extreme" and the one elf who was violent in her racist attitudes towards humans was exiled for exactly that reason.
And I've already talked about examples of Dalish Elves not looking down on City Elves elsewhere but to reiterate:
Hahren Paivel says that City Elves are important to the Dalish because they will teach them to understand the shemlen. Lanaya doesn't understand why they live the way they do but doesn't hate them. Marithari shows respect to the City Elves that greet her. Pol was welcomed to the Sabrae Clan when he first arrived. Merrill shows no contempt for them at all when she goes to live amongst them, she even tells Fenris that the Tevinter elves could go to the Dalish for help. That one shopkeeper elf in Denerim's alienage supposedly was helped by the Dalish after he escaped the life of a slave in the Imperium.
But City Elves themselves are not exactly the highest paragons of tolerance and friendliness to outsiders. They're fairly insular. And they have their own context for the slur "flat ear" - City Elves that leave the Alienage. Pol tells the Sabrae Clan that many of them don't look at Dalish Elves as real elves just like the reverse. Shianni treats all outsider wardens badly until she realizes they saved Soris (if they did). And sometimes they've been known to threaten humans with violence if they think they can get away with it (like Mother Boann).