How important is a good AIME score for college admissions?

That’s pretty much par for the course. Schools (with a few notable exceptions around the country TJ in Virginia, Stuy, Bronx Sci and HCHS in NYC, etc.,) are focused on getting the below grade level kids up to grade level, and for improving the performance of the average kids. Getting the 99th percentile kid up to 99.9? Not their mandate, or their problem.