Actually, playing non-meta champs actually can give you an advantage over playing a meta champ for several reasons:
-learning to play a non-meta champ means that there is less of a chance that your champ gets banned
-your opponents will likely not know how to play the matchup against your lesser-played champion, while you will likely get a lot of experience in playing against your opponent's fotm champions
-other players will likely play FotM champs because they are strong, and not learn their deeper mechanics, giving you a skill advantage with the non-meta champions you play as, especially if you main that champion almost exclusively
Does that mean always playing non-meta champions is always good? Not always, but there are some advantages to picking one that don't normally happen when you play a meta champion.