As their name implies, the Dreamlands are fantastic worlds that exist within humanity's dreams. Several stories by H.P. Lovecraft are set here, or were latterly relocated here, including The Doom That Came To Sarnath. Although not part of The Carcosa Mythos, some stories from the Carcosa Mythos occur within dreams or are dream-like, so it is possible that Carcosa could exist here. The Great Ones are the gods of the Dreamlands (and Earth's antiquity) and receive protection from Nyarlathotep.
A similar dream realm appears in Dreams of Shreds and Tatters.
An alternative view recognises great Cthulhu as leader in the Dreamlands, and taking into account his rivalry with Hastur, it is unlikely that he would leave Carcosa alone. It is a popular fan theory that Hastur's influence somehow disrupts the Dreamlands themselves, or at the very least his servants' entry to them (this is further reinforced by the already established fact that the Byakhee, Hastur's servants, are immune to outside mental influences by other creatures, so it can be easily argued that this immunity to mind-influencing effects extends to all of his servants).