In my opinion, no, it's not an insult. It's a term used when you're referring to someone who has the odds stacked against them, but there is an underlying amount of support that is there because there are hopes that they will win.

Betting Odds Explained\n\n The +  and -  are put in front of odds or lines, indicating the favorites and underdogs. The favorites will be marked with a minus sign, whereas underdogs have the plus sign. This is the case for all types of bets, including moneylines and point spreads.