Because teemo can take kills easy, especially with his poison, that can make the adc precious kills go to the support who doesn't need them, then the adc gets mad and for a good reason, as teemo overpowers the adc more and more kills may go to him instead.
I am not saying it's a bad idea but you said the low elo and the low elo is exactly where this happens. Also teemo other that a 1.5 sec blind provides no cc to the fight so he is a dmg rrelated support, makes it easier to kill steal the adc.
I think in the right hands he would make a decent support, i mean annie isn't much different. Lil hoe has so much dmg that ADC can lose many kills to her.
I just think ESPECIALLY in low elo an ADC wants a support more like leona, blitz, braum, alistar. Something tanky that doesn't possess much dmg so as an ADC you can 'ks' them and make sure the gold gies in the right hands.
Personally a leona or an alistar or a thresh is so much better and preferable to be my support, that i wouldn't even consider a teemo.