I think Lewis was making an important point about obedience and sin. If we understand the four signs to be like God's law for our lives (as many have pointed out), then the most common error we make is not outright rebellion. At no time did Jill stand up and say, "nuts to these signs - I'm going to do other things." She intended to follow them - but she let herself get distracted. She let lesser matters distract her from remembering and following them, despite Aslan's explicit warning that 1) they would prove difficult to remember, and 2) they would not look like she expected them to look when she came across them (which turned out to be true in all cases). She let everyday concerns crowd out remembering and obeying.
This is very similar to Jesus' warning in the parable of the sower in Mark 4. Recall the seed that fell among thorns, which grew up around the seed as it sprouted, choking it and making it unfruitful. Jesus explained this as His followers who became distracted by "the worries of the world, the deceitfulness of wealth, and the desire for other things." They were choked into unfruitfulness.
If we look at the sins we commit, it is very rare that they are outright rebellion. Far more commonly, we simply can't bother to be obedient, or are busy about lesser things, or are frightened. Just like Jill and the signs, we let smaller matters distract us, and before we know it, we're disobeying.
Having said that, I don't think Lewis deliberately intended to say that. I think he just wrote the story as his imagination "saw" it, and that lesson was embedded in it.