I get why most people are annoyed that he seems so stupid and ignorant. However, you're looking at it the wrong way. Think about it this way: you live in a world full of superpowerful magic. You've never encountered a monster and only heard rumors about them and you have no one to compare yourself to as you train to grow stronger on your own. You one day encounter a monster and kill it easily.
Obviously, you have no reason to believe that you are strong just because you killed something big in a world in which a little girl can ruin a fortress effortlessly. How would you be able to judge, when size obviously does not relate to power in any way in your world? When you quit your training camps, you were a weak scrub, so of course you have no reason and no way to know that you aren't a scrub anymore.