Picking Off-Meta is fine..
But not if you fail miserably with said pick because you have no clue how said pick works. It takes TONS of practice to get good with any champion that isn't meta, and if you haven't put that in, forget about making that pick work. Because the thing about meta champs is....that even if you're not anything special they will do a specific job more often than not which is why they are so valued.
With niche picks...you need to know EXACTLY what you are doing and what your goal is. If you don't you are going to fail even if you get 3 kills early and massive lane advantage.
You can't pick Vayne top as a counter without first being a master of vayne, you can't pick Shaco or Rengar or Diana Jungle without knowing eveyr little tip and trick that allows you to truly carry with them. You CAN pick Syndra/Ori/Lucian/any support/j4/Kha'Zix/Amumu/etc. and fuck up hard and still be relatively OK because their base kits just bring so much to the team that even without expertise, you will be providing something. Long as you do the basics.
Not so for picks like Malzahar, Shaco, Cassiepoia (or any mid mage that isn't either super long range or assassin) you're just gonna get fucked and contribute nothing to the team.
The exception to this is hard CC champions - these contribute no matter what unless you lack even a basic grasp of what this game is about in which case your pick isn't even close to the problem.