What does 'excited much' or 'stalker much' mean exactly, and which context are they used in? I don't get the usage of much after a noun or adjective. I often see this construction in comments, for example.
Mark Liberman of Language Log discusses the “X much?” idiom with a recent entry from OED (some emphasis mine, some examples omitted):
Liberman notes that the idiom uses much in novel ways: “Jealous much?” insinuates that the target is quite jealous, whereas the conventional “Are you jealous much?” would inquire as to frequency, not intensity. He also notes how people use “X much?” even for words that don't easily expand to a full question: “Ad hominem much?” or “Martyr much?”
In my experience, the idiom ranges from whimsical to critical to sarcastic. Sometimes it's a teasing accusation, and sometimes it's even a boastful suggestion that the target should be “jealous much.” A similar idiom, “You mad bro?” has gained currency in recent years for cases where the target is angry.
I think you are referring to a construction in which a characterization is presented in one word followed by the word much, read as a rhetorical question and intended as a criticism. It is not intended or imagined to be grammatically correct.
"Excited much?" would be "Do you get (this) excited often?" and is intended to paint the subject's enthusiasm as unwarranted and to suggest that it reflects on or represents the subject's general character.
"Stalker much?" is an attempt to portray behavior as stalker-like and would be grammatically written "Do you often behave like a stalker?"