Mandy’s introduced in the first season, having an attraction to Ian (Cameron Monaghan), which isn’t reciprocated because he’s gay. But she agrees to serve as his beard while he pursues a romantic relationship with her brother. In Season 2, she starts dating Lip (Jeremy Allen White), and the two have a rather tumultuous time. She comes in and out of the series at various points, usually not at a great place in her life, eventually disappearing entirely from the Gallaghers’ lives.
There’s no definitive answer on Reddit for who people think played Mandy better, partly because Mandy changes significantly between Seasons 1 and 2. Redditor u/-Green_Curtains- points out, “I liked both Mandy’s but I think her storyline took such a turn from season 1 I can’t imagine anyone but season 2 doing it.” Such sentiments were echoed by u/jollycanoli, “I think S1 Mandy was a really strong actress who gave the character a lot of likeability and vulnerability. But S2+ was a tougher Mandy, and the actress was arguably a more credible choice for those storylines.”
Ultimately, it’s a matter of preference, but both actresses did a good job with the material they were given. And, of course, there were those completely unaware a change even took place, like u/937Vibez writing, “ngl, I didnt even notice the actress had changed while I was watching the series.” Overall, it’s a pretty successful switch when a portion of the audience doesn’t even notice anything happened.