I got the exact same score and percentile as you for this April 2018 test. Maybe you are looking for advice from someone who scored better, but I'm going to offer some comfort hopefully haha. This comes from advice multiple mentors of mine and being friends with lots of people who have gone to various different grad schools (mentors from 3 different universities, friends from tons of different schools as I have done 2 REUs).
I do not believe that this score means you did not study enough. It perhaps means that you did not study correctly. Because of the nature of standardized tests and the time restraint of this particular test, the best way to study is not by comprehension, but by memorization and studying for the types of problems that will be on the test. The math subject test is a test where you have to study for the test, not to understand. You have to be able to immediately recognize the problem and immediately remember how to do that problem, a memorization strategy. What if you understand the concepts of a problem and could reason it out, say in 5 to 10 minutes? Well then you know and understand the material, but you probably won't get it right on the test or have enough time to do it. At the end of the day, because of the standardized nature and time constraint of the test, this is not a comprehension test, it is a memorization test.
As for your second question, no, I do not believe this means you are unprepared for grad school. I have taken and been successful in many graduate courses as an undergrad. I know people who have scored significantly worse on the math subject test that are doing great in PhD programs right now. If you were admitted to a program (like you are), then they believe you will succeed.
Congratulations on your acceptance. Happy doing math.