Is this a common slang in US? If this is common, what is the meaning of it? I found only one hit from google search. It's here.
As a native speaker of American English (Midwest and East Coast) I've never encountered the word dirted in slang or any other use. An appropriate alternate word would be dissed, meaning 'disrespected'. In the original context the phrase would change to:
In this context, hardcore means 'intensely' or 'severly', as in definition 2 here. It is worth noting that while the definition shows 'hardcore' as an adjective, it can also be used as an adverb in cases such as the above example.
Hope that helps.