For those who need more clarification here is a news article:
In terms of Overwatch, I feel it is important to remind players that owning an alternate account is permitted as long as you are playing to your best ability to gain to your expected skill level. Throwing or boosting accounts for any reason is not permitted.
Source: Serious question: Is smurfing cheating? - #5 by JeffreyKaplan
By my personal definition a “smurf” is having any account that you use to deliberately play below your actual rank often in a matter to throw matches but still find ways to outplay and frustrate both team allies and opponents.