It's shoddy writing because it's not made believable. Wakka has no reason to call her that, and later he never refers to her as either an apprentice or a summoner. It's not just towards Tidus, but towards everyone in the beginning. Heck, he may have called her the apprentice summoner in front of Tidus once or twice since he's out of the loop, but between everyone else it should have been Yuna or Lady Yuna. I personally blame the translators. Tons of honorifics are normal for the Japanese, but they do not work in English.