Most people don't like Teemo because they don't understand how to play him. They know the basics, but they don't know what makes Teemo strong against most top laners.
The biggest and most common mistake is following a concrete build. Riven got you down with a Hexdrinker? Sion got you down with a Spirit Visage? Liandry's just not cutting it? Get a void staff and sorc shoes. Shred that MR clean off. Fiora building a hydra? Buy a Seeker's Armguard and Morellonomicon. When she gets low, she isn't going to be lifestealing for sh*t and you have some good armor going.
This leads into the second most common point I see in Teemo players that don't fully understand what Teemo is all about. They see his skills, and they see the surface of why he might be a valid pick, but when it comes to practice things go south. Teemo is a knowledge based champion, and he's very good at it. His shrooms give vision, allowing you to have knowledge of the map in your immediate area, and also prevents ganks from going quite so smoothly. This is why you have to pay very close attention to your opponent's build so you can counter their items, as well as their moves. It's IMPERATIVE that you know your matchups. If you don't know what Riven does, where she has power spikes, where you have power spikes, when you should go in, when you should retreat... it sounds pretty basic, but for the longest time I had trouble with Fiora because I didn't know she could dash twice because her burst would just destroy me before I could do anything, even when I thought I could get away. I thought her lack of cc meant she would be easy to get away from. I was wrong, but now I have a lot less trouble with her now that I know what she does, and how her skills interact with mine.
Some Teemos also love to burn mana for no reason at all for some reason as well. I don't know why, but sometimes I see someone complaining about how bad Teemo is as a champion, but they constantly use blinding dart to poke, so they run out of mana constantly. I ask why they don't have shrooms giving them vision of key areas (tribrush, river bush, etc.) and they say "I don't have any mana because I have to keep poking."
Also, most people don't use his w to dodge things. You need to do this, and get good and shrooming defensively. I don't mean putting them where you know they'll go when you're being chased, I mean be ready; as soon as they jump in at you, stick one down and hope they get blown up (sometimes you aren't fast enough and that's... not so good...). This helps a lot with people that like to get into your face really quickly like Riven, Fiora, and Renectan.
So, is any of this new to you? Do you already do any of this? I don't know how you play, so I came prepared with as many tips and common mistakes I could think of, just to be sure I covered the bulk of it.
+ 
is pretty descent at tank shredding, and Teemo is an AP marksman, an should build similarly.
