ATAR is less important than knowledge of, and performance in, the subject you want to tutor. Then, there is ability to explain / communicate concepts, and the level you want to tutor. Plus, studies beyond year 12.
There are people with ATARs of 99+ who are terrible tutors because they can't explain in a way that others understand, or lack the ability to recognise where someone is struggling and how to assist. There are terrible tutors who think that giving a (the?) correct answer is all that is required, when sometimes assisting someone to deduce it for him/herself is much more useful.