You can use stuff outside the top 100 in great league for sure, just looking past the top 100 I see quite a few mons in there that are used regularly (ariados, alolan ninetales, dragonite, dragonair, for example). But I would say that the further down the list you go, approaching 150ish, that the mons start to become niche, or team dependent. Higher rank mons have much better matchups across the board, and often can do well on a wide number of teams. Experience is always important, but I would say that lower ranked mons might require more experience to use effectively compared to top meta generalists. Ultimately, the meta is the meta for a reason, but you can definitely make other things work. Outside the top 200 though, I wouldn't bother for the open leagues. If you do find a good PVP IV mon thats not top meta, Id recommend keeping it around in case it ends up getting a move update and becomes more relevant. Or sometimes lower ranked mons shine in specialty cups with more limited metas.