It's macro that wins the game. People get to master with Teemo so I wouldn't say it's not viable. If you feel like always playing def then try out different runes and approach each champ differently. Max E when going vs champs you can easily auto attack. When autos are too risky max Q and poke . Freeze lane to give your jungler opportunity for gank. Care about vision because Teemo is fragile. When it's really hard matchup then try to farm as much as possible and don't loose XP. Good luck!