People always seem to use filler as derogatory, mostly cause show filler (Anime/TWD/Etc) can be quite dull. MoP was "filler" as it did not fully relate to the overarching stories being told, it was not stories known from other sources. People are assuming BfA is "filler" because so far we have no story and seems like a bridge from one to another.
But it doesn't mean bad, MoP was great, story does not always make an expansion, this isn't a tv show where you're wondering if Glenn really died and they're just stringing you along with Morgan filler.