Tia's maturity is low and not the most intelligent. This is a undeniable fact.
But honestly, those qualities are what makes her the perfect heroine for Cecil....that pure-heart kindness of hers coupled with the above two "flaws" is what saves all of the other characters and spur their desire to be by her side in this manga, and I just can't see what she accomplished turning out the same way if she was a mature and smart woman. I at the very least would have found it far less enjoyable. But well to each their own.