The official online sources say I am not allowed to enter Germany in general (b/c not from EU, or from a non-EU country on the non-restricted list), but "transit" is listed by the same sources as an exception that would permit entry.

I have my tickets for Ulan Bator printed out. Aside from air tickets, valid passport and negative PCR test results (which I'll also have printed), no documents are required for entry to Mongolia.