Thank you for contacting us about Low Baller. I will be happy to clarify this!
Regarding Low Baller, I'm sorry but Jinxed is not a round where you can unlock it. You will need a round where you have to score points because a 1-0 win on Jinxed means that there was one person left to infect.
If you will ever win 1-0 in modes like Fall Ball and it still won't unlock, please contact us immediately and we will be able to unlock it for you.