Any Mass Effect DLC missions must be compared to the Lair of the Shadow Broker DLC for ME2, which not only served as a bridge between ME2 and ME3, but also drew from story threads first introduced in the original Mass Effect. It also filled in some of the events around Shepard’s death and return in between ME and ME2, and built on Shepard’s relationship with Liara in a satisfying way. Skipping Shadow Broker would detract from many story points in ME3 and make Liara’s brief appearance in ME2 feel like a strange dead end.
Neither Omega nor Citadel fill in gaps the way Shadow Broker does, so if we judge them based purely on their contribution to the overall narrative, both come up lacking, even in comparison to earlier DLC releases for ME3. Leviathan, the previous DLC, added some significant Reaper backstory, and the first add-on, From Ashes, offered an additional squad-mate, Javik, whose story fills in some nice background about the Protheans, the Reapers’ penultimate foes.
However, Omega and Citadel are both nicely written stand-alone adventures. Citadel in particular is a worthy addition, and suggests that Shepherd and crew would do nicely in a sort of Ocean’s 11 in space spinoff series, in which Shepherd has to crack wise and infiltrate parties before jumping around darkened warehouses.