First appearances were not to be trusted. But no one really thinks that; one shouldn't judge a book by it's cover, and that's what happens. You judge someone as a prep, a nerd, an idiot, and they dub you the same way. All because of how you look. You might be what they see you as, but you're not only that. Some days you're a prep, some days you're a nerd, some days you're an idiot. Like a kaleidoscope, you change every day. You could be the same thing, but not always.
And so when L first saw Matsuda, he initially thought that Matsuda was an intern or one of the mens' son wanting to play investigator. And then Matsuda opened his mouth, and that first impression changed from intern/son to "idiot."
As for Matsuda, his first impression of L was "weirdo" to "smart!weirdo."
And while the "second impression" was more accurate than the first, it was not the only appearance the two had. Matsuda was not only an idiot, he was comic relief, he was the puppy of the group, the loyal member, the optimistic one, the one who everyone took their frustrations out on. L was not only a smart weirdo, he was a cake-eater, he was the one who made people diabetic just by having them watch him chug down the sweets, he was the leader, their very own lie-detector / calculator / laptop all in one.
A first appearance was a glimpse to part of a personality of a person. A glimpse was not all someone was.