I have a smurf account and let me tell you something
After level 5, my quickplay MMR adjusted itself so I was playing with diamonds again, (my actual rank).
My first placement was with people around 2000 SR. After that one game, my next game as with plats and diamonds constantly and I felt like I was playing at my own rank.
I placed around 2950.
Took me two games to get back to diamond and literally end up where I was on my main
I used to be like you thinking Blizz should ban smurfs. After trying out for myself, I've realised it's insanely quick for the SR system to determine where you belong. It really isn't a huge issue.
If by smurfs you mean people that deliberately throw to stay at lower ranks, then yeah. Okay. That's a dick move. But people just creating a second account and then play normally? You won't meet em for more than one game. Max.