There are definitely plenty of people in roller derby who couldn't skate at all before starting and hadn't done a sport previously, but it's definitely a steep learning curve (just learning how to skate and also learning how to play derby). I thought it was great at the beginning that Mariana struggled to stay upright and wasn't immediately the best skater, because that's very realistic -- I definitely spent way more time than that on the floor at my first fresh meat practice. I am a little worried that she's going to all of a sudden be a great skater and player. I think it would be a much better storyline for Mariana to (realistically) struggle a lot with learning the skills and how to play but put in the work to improve even though she's not immediately great at it, because when it does start to click after all your hard work it feels amazing. It just seems like, from a storytelling perspective, she gets good quickly and becomes a jammer she's not actually going to learn anything about teamwork and how to not be good at something.
They also kind of made it sound like there's only one jammer per team in a game, but most teams have a rotation of three to four. You do have to be strong and fast with good footwork, so most newbies are not going to be very successful right away.