Black Comedy: There's a good amount of this in the movie. A good example would be when May, having kept Lupe, Polly's dead cat for several days, decides to spray the body with air freshener to stop it smelling.
Blood Is the New Black: May returns to her apartment absolutely covered in the blood of herself and the class of blind children after they accidentally break Susie's case and cut themselves on the broken glass.
Companion Cube: A darker example - May starts out (apparently, we don't know for sure) just pretending Susie is real, but then she starts to actually believe it. Oh, and of course there's Amy.
Then there's the corpse of Lupe. May hugs her, strokes her, and talks to her, until she decides to keep her in the freezer.
Creepy Doll: Susie, a little bit. Most notably when you can hear her whispering, or the glass on her case cracking - not that these are real, this is all in May's head. She certainly looks very creepy, too. Also Amy, the life-size doll made out of body parts.
Emotionless Girl: May, once she's decided to make Amy. Although when Amy doesn't come to life, she becomes very upset again.
Eye Scream: May gouges out her own eye with a pair of scissors in order to finish Amy.
And prior to that, she starts clawing at her eyes after having worn the same set of contacts for several days, a horror that most contact lens users have run into before.
Fetish: May appears to have one for blood (especially after viewing Adam's film), and Polly promptly develops a fetish for cutting after May cuts her thumb with a scalpel, requesting that she do it again.
For Halloween, I Am Going as Myself: Sort of. May is able to get away with having blood stains all over her forearms and lugging a large, heavy and normally suspicious-looking wheelie case around in public, as she commits her murders and transports the bodies during Halloween.
There's May's focus on certain body parts for each person (Adam's hands, Polly's neck, etc.).
Her homemade clothes, many of which are sewn together out of old outfits.
There's her comment to Susie about how Susie has always "seen [her]".
Her job at the animal hospital.
Her complete lack of any disturbance at anything violent or gory and thinking that Adam's movie was "sweet".
Her mother's comment that "If you can't find a friend, make one."
Polly calling May "doll" as a term of endearment.
May gouging out her lazy eye at the very beginning.
Freak Out: May has a huge one after Lupe's body is discovered in her freezer. She snaps and murders Blank, the guy who made the discovery. Following the murder, she suddenly gets the idea to make her own friend...
Genre-Busting: It's a weird, creepy, depressing romantic dramedy-ish... thing... that ends up becoming a slasher/Frankenstein movie for the last twenty minutes or so.
Innocently Insensitive: Polly sleeps with May under the impression that it's a casual fling between friends. May, however, thought it was serious, and so is absolutely devastated when she finds Polly hooking up with Ambrosia.
Madness Makeover: Interesting example. Contrary to the usual pattern of becoming more sloppy and disheveled as one goes crazy, dowdy May becomes much more stylish and sexy as she plunges into homicidal insanity.
Sanity Slippage: May, slowly and steadily, right up until she kills Lupe (it is debatable as to whether she actually meant to kill her or not) and keeps the body. Then she moves on to humans...
Self Harm: At one point May cuts into the tip of her thumb with a scalpel, apparently as a form of relaxation. She then tries it on Polly, who unexpectedly finds that she likes it.
Serial Killer: By the end of the film, May has killed five people and one cat.
She's Got Legs: Ambrosia again. May certainly thinks so, too, and uses them as parts for Amy.
Shrinking Violet: May's only comfortable social interactions are with Susie, and she's so shy that she can barely talk in the presence of other people at all, even when they're clearly interested in getting to know her. However, once she resorts to murder, she becomes much more confident.
Significant Anagram: When deciding on a name for her creation, May rearranges the letters spelling out her own name from the broken ashtray to form "Amy".