Fast Pass is supposed to guarantee that your wait will be shorter relative to the standby wait, usually by letting in far more FP people than standby people at the point where the lines merge.
The problem with character meet FPs is that characters have lower capacity than any ride, even with the extra magic in Princess Fairytale Hall, and thus the lines move more slowly. So if there's already a line built up by the time you use your FP, even with a FP the wait will be long.
I used one midafternoon on Rapunzel last trip, and the FP wait was 45 minutes with a 90 minute standby wait. Faster than standby, but still a much longer wait than what we usually associate with FP. And on Valentine's Day, when Rapunzel and Eugene met together for the first time in forever, the line reached 90 minutes FP and almost 3 hours standby. Luckily I had the foresight to do that one at rope drop!
I predict that demand for Anna and Elsa will be similar to the Rapunzel and Eugene situation every day. So I'd still recommend doing them at rope drop, but if you can't, a FP will probably still be better than the current situation in Norway.