I'm actually surprised AOC isn't doing better. The fact she isn't polling ahead of both Bernie and Warren shows just how much her political star seems to have declined.
I'm not sure why you think it means she's declined. When has she ever polled ahead of Sanders? In any case, her standing depends on who's included in the poll question. In this poll by Echelon, taken in a similar timeframe but without Biden and Sanders listed as options, she's in second place after Harris:
https://talkelections.org/FORUM/index.php?topic=484843.0
