I never felt that Goodbye, Lenin was Ostalgie because the GDR Alex recreates for his mother seems to be purposefully build from the ideals she displayed herself, not from the GDR as it really was - and that's not only something the narration does, I think it is Alex's way of dealing with the fact that no matter how hard he tries, she's dying. In preserving this "ideal" GDR he also tries to preserve her. (And of course, the act of pretending also plays a role in the movie itself, for all of the characters, since the truth hurts so much worse.)

ETA: So maybe you could accuse the movie of melancholy over lost socialist ideals?