Troll picking =/= off meta. Reasonably speaking, there are two ways of playing 'off meta':
The first is positioning related. For example: Playing Garen as a jungler is off-meta. It's off meta because his jungle pick rate is non existent, he does not have any (commonly known) strengths for jungling, and he's not designed with jungling in mind / even as an aside. As such, picking Garen in the jungle role is strange, and out of meta. Other examples of this could be: Bard mid, Zed support. It is worth noting that 'off-meta' positioning picks generally do not fulfill all of the 'criteria' for a role (Garen jungle ganks would be pretty weak), but are often times passable / decent at another aspect (Garen jungle would have a decent clear speed and semi-decent jungle sustain).
The second is build related: For example, full AP Shyvana is off-meta. It's off meta because Shyvana is demonstrably designed to be a tanky AD bruiser. This is something Riot has said. However she still has AP ratios, so she can benefit from AP items, and as such playing her as full AP is not 'useless', although it might be suboptimal. Other examples of this include bruiser / AD Akali, full AP Cho, bruiser Nid, and others.
It is worth noting that sometimes 'off-meta' turns out to be insanely strong, in which case it actually becomes meta where as the 'norm' then becomes off meta. Good examples of this include Jungle Nidalee, AP Nidalee, bruiser Akali (pre and post rework), ADC Kennen, Full-AP-Mid Kog'maw, Full-AP-mid Janna, ect. It's also not uncommon for these builds to get absolutely dumpstered with nerfs.
Now, let's talk about troll picks.
Troll picks are basically the same two 'off meta' playstyles, but with malicious intent:
Positioning related: Jungle Yuumi would be a troll pick. This is because a large portion of Yuumi's kit cannot be used in the jungle roll; not that it can't be used well / doesn't play into it, but it straight up cannot be used. Also, unlike other 'uncommon' jungler picks (Karthus, Garen, Renekton), she has absolutely no tools to give her a strong jungle clear. This flags her as a pick which is weak in every single area that she must fulfill for the desired role.
Build related: Annie has the ability to auto attack from range (something not unique to her). Beyond that she has absolutely no synergy with AD or attack speed items; every single one of her scalings is AP, and she gains no reward (bonus AP damage, tagging targets with debuffs, 3-hit-procs, ect) from building such items. As such, building ADC Annie would be a troll pick. I do not mean that playing Annie as the 'ADC' role in botlane, and building AP would be a troll pick. That's off-meta. But building her as full AD & AS would be a troll pick, because she has absolutely no synergy with AD (Renekton has more synergy with AP than Annie does with AD).
Beyond that, there are a few outliers. Something like a support-top at the cost of solo bot would depend on what's being played. A Leona 'supporting' a Darius, and playing as a regular support, is merely off-meta. A Zed 'supporting' that same Darius, & attempting to take every single CS out from under him, is troll picking.