Interesting theory. Makes me wonder why she wanted to duel Toot and the others for the Pizza, though (see below).
Ive been thinking that maybe it has something to do with the Fae views of Gifts and Repayment. We know that the Fae are incapable of accepting a gift without giving something equivalent in return, and given how highly they regard pizza it's no surprise that Toot and Co have interpreted that as Lifelong Service and Loyalty. When offered food Lacuna, by contrast, asked for weak tea, curdled milk, and one of the most nutritionally empty foods known to man. Such foods would have less value by far that The Pizza. If however, she dueled them for the Pizza, then it would be the spoils of war and not a gift....
You can’t do it!” she cried. “You can’t just give them all that pizza! Do you have any idea how much harm you’re doing? Can I please fight now?”
“Whoa,” I said, leaning back and holding up my hands.
“Hey, shortcake,” my brother snapped. “Back off.”
“You aren’t important,” Lacuna declared to Thomas, evidently dismissing him entirely as she turned back to me. “I wrote down everything just like you said and now they’re going to get that awful pizza all over themselves without the least regard for properly protecting themselves, and I’m going to fight them for the pizza!”
PS Werent the original Tooth fairies little things that would eat your teeth, more of a tooth troll or something?