keep in mind that his friendliness isn't stupidity, it's just the way he is-- he's kind of a shounen protag in the sense of believing that friendship can overcome anything. when you insult him, he pretends that you're talking about yourself because it leaves an opening for him to make those overtures of friendship. unlike sans, he's genuinely willing to spare frisk in a no mercy run no matter what they do; he admits that he was really scared! but he wanted to believe that he could talk them down with friendship. he's honestly proud of them if they do spare him ("Wowie! You didn't do a violence!") and if they disregard that and kill him anyway, he expresses regret for not being able to help them.

he's not an idiot. he's a really, really good guy.