Depending on your worldview, certain Slaaneshi worlds could be a utopia.
That to me, would be impossible, Slaanesh is the God of excess and by definition an excess cannot be satisfied and by definition an utopia is a place where everybody is satisfied and generally happy. A state of contentment is anathema to Slaanesh. The Gods of Chaos are by nature self-destructive and thus no utopia can be built around them. Even the Eldar who created Slaanesh were not happy prior to the Fall. They were miserable and they are the species most likely to "enjoy" life as a Slaanesh devout.
Does a utopia mean contentment (and therefore stagnation)?
I would say that stagnation isn't a utopia, so a place where "everyone is content" couldn't be a utopia by definition. I would say it is a place where everyone is happy (and, as always with humans, what makes them happy changes as they achieve it)