With Lemmy there's distinct difference between the frontend/UI & backend (the application itself, & data storage). Lemmy is primarily the backend, but they allow anyone to create a frontend (or mobile app, browser extension, etc.) to interface with it. Lemmy does come with a default web UI which sucks, and this is where Kbin rose to initial popularity with a slightly better web UI. Now there is a robust ecosystem of apps like Voyager or Sync which are quite good and far more enjoyable to use, even when compared to Reddit's official app.

Good news is the web UI can change, and you can experiment with other federated instances which uses different web UIs.

Lemmy's main problem is a lack of user base and content. I've lurked daily on Reddit for 13 years without an account, but with Lemmy I'm choosing to vote, comment & post to help get the "ball rolling". Reddit had the same problem initially, and the creators & friends busted their asses with alt accounts to give the impression of a larger user base... Until Digg imploded