Yay! I created Kia and one of her "quirks" (Coffee Cake, hehehe) is that she is a scatterbrain! She likes to say that she's Wonder Woman, because she always wonders where she last puts anything! Always day dreaming, but can be serious when she wants to (which is not very often, lol)
If you dont want to use that one (I like both of these equally) you can say she's too optimistic. Like she always makes jokes and laughs/giggles at a serious time, and mostly everyone thinks it is annoying or something.
So, either
•Scatterbrain (Head up in the clouds, wonders 'bout everything, not very serious
•Too optimistic ( Laughs/giggles about everything, kinda annoying, not very serious.
Edit: While I was writing this I remembered of that one video that FunHeart posted on KN about this lady wanting to divorce her husband, John (he laughs so much... its creepy