I agree that the Zelina twist was likely a later plot twist, so the curse likely wasn't intended to take out another powerful user of magic.
The primary purpose of the Snow Queen at this point was to make Elsa feel isolated, so that she will be easier to manipulate for her scheme/sisterhood development. All the townsfolk know someone is in there town who has already frozen and attacked Storybrooke, possibly through a lack of control of their powers. Only a few who witnesses Elsa being unable to take the ice wall down know that someone else must live in the town with similar powers. Mary Margaret "helpfully" tells the townsfolk at her meeting about Elsa, stating she is not a danger, and then in true dramatic fashion Marian freezes. Ultimately, this should increase the distrust the townsfolk will have of Elsa. Marian being the target was just opportunity as she happened to stop by the ice cream shop the day after Elsa showed up; the ultimate goal did not require any specific individual, just that someone was frozen and the town knew about it.
As for why Marian was picked from an out-of-story perspective, it provided ample opportunity to continue the romantic dynamic between Regina and Robin Hood without interference from Marian (rather just using her as an overarching "my wife still lives" block from Robin instead of someone who could discover the relationship flat out). She was also a minor character and the writers weren't going to freeze someone like Emma or Killian and have them sit in ice for a couple episodes.