How is Teemo a top
Teemo has a solid lanning phase against low-mobility melee champions (Fighter/Tanks).
Basically, anyone Kalista is good against, Teemo is good against.
Range gives him a solid advantage concerning creep aggro, not getting pathing blocked, and tower protection for the early stages of lanning as well - all of those things are actually a pretty big deal early on.
Teemo's on-hit poison is also really strong, applying the equivalent of a Q nuke for no mana as long as he can just get an AA off and back away. If you can play a poke-tactics game with Teemo, you can usually win lane against a wide range of melee champions in general. And his blind and MS boost give him a bit of wiggle room for pulling off hit-and-run strategies well, as you can negate just enough retaliation damage and re-position just fast enough that trading against a solid Teemo can become a big risk that he might capitalize on.
If you try to face-tank with Teemo via Blind you'll get obliterated though. Teemo's #1 weakness is hard-engage, so if you bide your time as a melee than relentlessly attack him with everything via a full-commitment fight, that's how one deals with Teemo.
Just - if you play Teemo, you're David and you're up against Goliath. You resort to every underhanded, indirect tactic available to kill the giant you're up against.
Is he the worse champ cause his kit doesn't fit any particular meta team.?
Teemo's the worst champion through being the most annoying, what with all those irritating indirect guerrilla warfare tactics he uses.
His unique kit set-up pretty much guarantee he'll never be the worst champion in terms of viability though, as he doesn't suffer from being a weaker version of champion X, and having a pretty unique kit allows Teemo to have a monopoly on a specific strategic niche.
Nobody does guerrilla tactics quite like Teemo.