If Mordin survives Tuchanka, is he present in the Citadel DLC?
Solution 1:
No - regardless of whether Mordin survives Priority: Tuchanka or not, he will not be present during the party in the Citadel DLC. In all instances, the only Mordin content is the datapad next to the bed when you wake up the next day.
In the instance that Mordin survives, it is expected that he is in hiding, and that walking around the Citadel would expose that he is, in fact, still alive.