Sort of. They've worked in mysterious ways over the years. They fucked with DES back in the day (specifically, the S-Boxes, which are big tables of data used during calculations), but evidence since then suggests that they actually improved it. However, they also seem to be responsible for keeping the key length short, which meant it was inevitable that computers would eventually be fast enough to break it (which it was by the mid to late 90s).

The NSA has a dual job. They want to break encryption, but they also need to protect US secrets. Since industrial espionage is a thing, that extends to protecting the secrets of private sector companies. So they sometimes want to improve encryption, and sometimes want to put in backdoors. If you call up someone in the NSA, there's no guarantee the person your talking to will be on your side or not.

Fortunately, cryptography in the public space has advanced substantially since DES was invented.