Gambling on losing teams is a legitimate strategy, but more than betting on losing teams I think it is evaluating the risk/reward scenario with a specific bet. Sure, there might be a team that is likely to win, but maybe it pays a penny to the dollar. Basically, winning that bet doesn’t make a difference. I get more entertainment out of betting longshots and making sure the reward is worth the risk.
If the popular team or fighter become an underdog, I think it's good to bet them because usually they have a good chance to pull an upset. But for a mediocre team or fighter, I don't think it's wise to bet them because they mostly lose, only few times they win.
I never want to bet on a huge underdog, it's just like a lottery where the chance to win is near to zero.
But before you place a bet on the underdog team, you should check how likely they are to win so you won't be disappointed or upset. If their odds are not big, you don't need to place a bet and just watch it. Watching them compete is also a form of support for them. After all, they also wouldn't know how many people supported them in that match. We must be wise in placing bets and not waste our money on bets. But it's okay if you want to place a small bet for your team as a form of support so that if your friends ask you, you can answer by placing a bet for your team.