No, tanks are not OP.
Tanky characters benefit from extra resistance or health regain, but this isn't to say that they're unkillable.
If you play an ADC or AP, there are items that can rip away %'s of HP in addition to normal damage. Build some slow into your auto-attack or spells to kite them easier. Maybe build an item like Maw of Malmortis if you're ADC, so you get damage and some MR, or Frozen Heart to have armor, HP, and mana. Don't focus on just pure damage items.
Likewise, consider NOT doing 21/9/0 all the time. Shift some extra points to defense, or use a defensive summoner spell like Shield or Heal instead of ignite. I've found Heal tends to be useful in helping your team in teamfights, rather than relying on Ignite to reduce 1 target's healing and maybe kill them if they're getting away.