Was Eve’s sentence at Gen 3:16 not unjust, if her main failing was naivete?

There are two things to say here. One is that Eve’s main failing was rebellion against God and rejection of his law, not naivete. The other is that naivete, as it turns out, is pointed out occasionally in the Bible as a significant failing, not just in the case of Eve. Jesus urges his disciplines to be “wise as serpents, and harmless as doves.” (Matt. 10:16) Proverbs is full of sayings about the dangers of being “simple,” and Paul warns sternly against falling prey to false teachers.