Mlem dev here. Tagging is a feature implemented by Voyager and some other clients, Mlem doesn't have this feature. It isn't an official feature of Lemmy - clients store the tag information locally on-device rather than on the Lemmy server.

In Lemmy 1.0 (an upcoming version) they're adding support for adding notes to users, and Mlem will be adding support for it at that point.

TL;DR switch to Voyager or wait for Lemmy 1.0.

[–] orca@orcas.enjoying.yachts 2 points 1 day ago

Woah, I feel like I summoned you! Thank you for this app! I use it daily.

I had a feeling it was an app-centric feature. Thanks for the info!