Yes, Shub-Niggurath along with a host of other characters came to life within the stories of HP Lovecraft. Currently there are cultists who focus on these creations, for example, the Cult of Cthulu. This group has created an entire paradigm from the legends of these beasts.
Whether or not they are accepted as mainstream does not matter; they like many other fringe practices in magic are focused on the principles of Chaos magic which allows the use of traditional or created symbolism to achieve the desired effects.
It is also important to consider that the tales of shub-niggurath are an example of the use of mythology along with spirituality and magic. Symbolism is a very important aspect of magic; it is the language by which the elements are combined and put into action through symbolic methods.
What did Shub-Niggurath represent to LaVey? A comparison to the Goat of the Witch’s sabbath, combined with the imagery of the Chaotic Shub-Niggurath of Lovecraftian fantasy. This symbolism eluded to the nature of the ritual, an initiation into the secrets of magic, as the leader of the rite, [Shub-Niggurath] spoke the words of the ritual. I do not imagine that LaVey and others were worshiping a goat-god, but they were glorifying the symbolic aspects of what it represents.
Is Shub-Niggurat as real as Satan? That depends on your vantage point, your perspective. From a symbolic view neither Shub-Niggurath or Satan are ‘real’ in the sense that that have a corporeal or [non-corporeal]form. They are symbolic entities. From a spiritual point of view, Shub-Niggurath and a host of other beings exist in a spiritual form, as there have been many who have contributed to their existence over time; much like the creation of gods through a societies worship of a central deity figure.
Symbolic or not, the image of Shub Niggurath is familiar to satanists for a good reason. It represents manifestation and creation, and complete destruction in one single force.
Feel free to read the original essay at the spiritualsatanist site: Shub-Niggurath