Apologies for not bothering to research this question in advance.
It absolutely is. This is known and explicitely called out in Tor's design:
> A global passive adversary is the most commonly assumed threat when analyzing theoretical anonymity designs. But like all practical low-latency systems, Tor does not protect against such a strong adversary. Instead, we assume an adversary who can observe some fraction of network traffic; who can generate, modify, delete, or delay traffic […]
What gives people, especially in the west, the confidence to use it at all, given all of the fusion centers and likelihood of parallel construction? Perhaps this is a rhetorical question...