One of the things you really want to do early on in your script is establish the genre. Juno shouting this line at the dog, Banana, does exactly this and the audience now knows they are rooted in a comedy – “Jeez Banana shut your frickin gob okay?”.
“This is the most magnificent discarded living room set I’ve ever seen” – another witty and funny line, foreshadowing Juno’s personality which is going to be revealed to us in the next few scenes.
Juno remembers the night with Bleeker as magnificent and nothing to regret. This is a very important and subtle setting of story tone, hinting to us that we are going to be given a lighter positive look at the potentially heavy subject of teenage pregnancy.