I’d say it’s an alternative to Instagram, but not Flickr. A huge distinguishing feature of Flickr is that it keeps high-quality versions of uploaded photos. Instagram and Pixelfed compress to a low quality. Pixelfed’s UI also suggests that it’s more of an Instagram replacement, i.e. more about the social media aspect than about the photos. The photos are just an excuse to do social media.
If the main federated alternative to Twitter is Mastodon, the main federated alternative to instagram/flickr is definitely Pixelfed.
https://pixelfed.org/