>>54717006it truly depends on you. doing the teal mask during the main campaign gives you access to new mons and also the possibility some (heracross, applin, ...) much earlier and easily. You also get Ogerpon to use for your campaign.
If you do the teal mask after the main campaign then the lelvel of the wild pokemons and trainers will be higher (from lvl 55 to ~75 at the end). some trainers battles get more challengiung (for a pokemon game) with different movepool and the addition of held itmes.
As someone who did the teal mask during and after the main game in different playthrough, I would recommend doing the DLC after you completed the main game-desu
