I think this isn’t really mlem’s place to implement yet, as I haven’t seen it on any major Lemmy instance. What I have done (and did so with Reddit in the past): create different accounts for a collection of different topics, switching between accounts is fairly easy in mlem and wefwef for that matter.
Sure, I didn’t meant to discourage any development of grouping communities, I just don’t think developers will risk a full featured solution just for Lemmy at large to develop something different, they need to reconcile in the future.