To answer that I think we need first to ask ourselves two questions:
1/ Why do we have so many cultures in the first place?
2/ Is the reason why applicable to other planets?
1/ We have different cultures because us humans have continuously expanded our habitat after having outgrown it (because, mainly, of population surges). With expansion comes geographical separation and impossibility to communicate (pre-internet!).
Since the concept of evolution isn't just a biological one - as Richard Dawkins theorized and even dare I say proved, there is "sociological" evolution - we have to assume that any culture is constantly changing and evolving.
For example, languages shift at mathematically predictable rates - it's not just a question of adaptation. But then there's the matter of adaptation to new environment, etc. Wars, interaction with some peoples rather than others, alliances, religions etc.
All of these (and more) are the reasons why we have different cultures. In the end, peoples who are separated coninue to have their own evolving cultures, and in case of no communication, there is little statistical chance for them to evolve exactly the same way.
2/ If we're talking about a life form that we would be able to communicate with, a life-form that we recognize as close enough to our own that we can consider them "intelligent life-forms" by our standards, they have to have "evolved", and follow "evolution" in a way that is close enough to ours that many principles apply.
Is it possible to imagine a recognizably intelligent culture that would 1/ not multiply and outgrow its habitat? b/ not expand when it did? c/ still be able to communicate continuously when they did? d/ stay exactly the same all the time (no cultural, linguistic shift) yet be able to evolve technologically to the point we could communicate with them?
I think the answer to that is no. So the answer to the broader question "can there be a monoculture" (even more so is it the norm) is a resounding no in my opinion.