If you do poorly on the math exam, would USNA reconsider your offer?
USNA isn't looking for reasons to reconsider your offer.
Look at it this way, by getting an Appointment, USNA has made the preliminary judgment that you have something they want - the ability to succeed academically and the character/leadership potential to become an Officer in the Navy or Marine Corps. You have also beaten out thousands of other candidates in a highly competitive process. Once the Appointment is offered, the competition is over and USNA wants you to succeed.
I don't recall the ABS, or even know what it is,,,,but highly unlikely that any one time, one shot test is going to change how USNA looks at an incoming Plebe as long as you make a reasonable effort on the test. Of course, if someone completely blows it off and makes no effort, that is an indicator of another issue, not academic qualification.
Bottom line, if you keep doing what earned you the Offer of Appointment in the first place, you shouldn't worry.