Lemmy (like Reddit) is about topics, and communities are formed around them. Mastodon (like Twitter/X) is user-centric, and discussions are formed around them. They are simply two different approaches to the social component.
The nature of these social networks makes one more useful for already known users/organizations and less for the common user. In fact, if you want to follow a particular person, Mastodon is more useful, but if you want to talk about some topic in general, Lemmy will always be superior.