League-of-legends – Why can role icons be obtained by champions who shouldn’t play a certain role

Enemy: Heavy AD - Rammus, Malphite, Amumu

~ Rammus' passive naturally scales from armour, giving him higher auto attack damage

~ Malphite has the ability to slow auto attack speed of enemies

~ Amumu's ultimate renders those enemies affected unable to attack or move for two seconds. Additionally his ability, Tantrum, reduces physical damage received by a flat amount (calculated after the reduction from armor), and the ability's cooldown is reduced by 0.5 seconds for each time Amumu is hit by an attack.

Enemy: Heavy AP - Galio, Mundo

~ Galio naturally scales off of Magic Resistance

~ Mundo has amazing HP regeneration, something that is hard to counter for casters, especially seeing as if they have no anti healing spells (such as katarina), the only feasible counter is the summoner spell ignite.

Enemy: Heavy AoE - Maokai

~ Maokai has an damage reducing AoE ult, a great way to counter enemy AoE spells

Enemy: Tanky Team - Cho'Gath, Mundo

~ Cho'gath gets seriously beefy if his ult has been used well. It can give tanks a big surprise due to the only tank having true damage.

~ Mundo with his health regeneration is going to be a very hard guy to take down if the enemy lakes high DPS. His Infected Cleaver ability also deals magic damage equal to a percent of the target's current hitpoints, making him an effective harasser and damage dealer against many tanks.

Enemy: Low mobility - Singed

~ Singed has extremely impressive mobility and can be an amazing initiator / disruptor with his move speed, AoE slow and fling.

Enemy: High mobility - Shen

~ Shen has a global ultimate that can directly counter enemy global ultimates, such as TF's and Pantheon's, shielding the target they are attacking and teleporting to their side to assist them.

Enemy: High CC - Alistar, Mundo

~ Alistar's ult removes all CC when cast and can be cast through stuns, silences, and the like.

~ Mundo's Burning Agony ability reduces the duration of stuns and slows on Mundo by up to 35% at level 5.

Friendly: Heavy AoE - Amumu, Galio, Nunu

~ Amumu has an AoE root, which sets up AoE ultimates on your team perfectly

~ Galio has an AoE taunt that damages. Nearly as good as Amumu's root, perfect for setting up AoE ultimates

~ Nunu has a -devestating- ultimate if used correctly. In combination with some AoE CC, his ultimate slows and burst huge damage if allowed to channel to the maximum.

Friendly: Lack CC - Alistair, Cho'Gath, Amumu

~ Alistair is a CC machine, his stuns/knock backs are not effected by any CC reduction

~ Cho has heavy CC, with his AoE knock up and silence.

~ Amumu has a stun with a relatively low cooldown (however it is a skillshot) and an AoE root, with a pretty large radius

Friendly: No jungler - Nunu, Amumu

~ If you need to jungle as a tank, Nunu is probably the best roamer and jungler combined, Amumu is the second most capable jungling tank.

Best initiators - Amumu, Malphite, Rammus, Shen, Singed, Galio

~ Amumu - Bandage toss into the enemy, and initiate fight with ultimate ~ Malphite - AoE stun, with a good range, and an auto attack speed debuff ~ Rammus - Roll into enemy carry, stun, taunt them into your team ~ Shen - Shield, AoE taunt. ~ Singed - Flip a squishy into your team and AoE slow the enemy ~ Galio - Flash and ultimate into the enemy

Oh course the tanks I have mentioned can fill the role of tank vs any type of team, only they are specialised in the areas I have mentioned.

The short answer is no. You are always in a better position if you are using a champion whom you are good at playing.

If you pick a counter you are not good with, you will have to try and adapt your playstyle to a kit you don't know, create an item build path you haven't used and played with before, and try to figure out exactly "how" your kit counters their kit.

For instance, someone picking Talon mid vs a squishy AP mid would be at a serious disadvantage if they didn't understand how Talon's burst combo should be used. If you go in on Brand with no understanding of how to quickly get Talon's damage in before Brand can start his combo, you will die.

tl;dr: If you are trying to counter a lane, rely more on your ability to play a champion well than which kit counters your opponent.