Is 12 considered a good handicap??

12 is very acceptable and perfect for enjoying your golf if you a bit crap. Not that I know anyone like that.
Good is when you get into single figures, but stay there only 3 days
Very good is when you get to Cat 1 (5.4 or below)
Exceptionally good is scratch and you need to get a life or go for pro
Outrageously good is a + handicap.