As an Englishman I can say 'dumb litty' holds absolutely no meaning for me at all, so I guess it is some kind of American slang. A quick google search gave me this from urban dictionary:
A phrase privileged white kid douche bags use to try and sound hip.
"Yo, I heard that concert was dumb litty!" -Stuart
Some further research suggests, that is comes from the phrase 'dumb lit' which apparently means to be drunk or high. So yes It appears your translation of "Extremely awesome, extremely wild, very good" is pretty accurate, although I would add that it appears to imply that it was awesome because you were high or drunk.