William of Rubruck’s accounts of his time spent with the Mongols corroborates the accounts of other explorers like Marco Polo, as well as historical research, which say that the Mongol Empire was a complex, multiethnic, multi-denominational polity.
Be careful about making judgements about the relative levels of “civility” of non-Western peoples, as is implied in your question “Are they wholly barbaric or also civilized…?” The idea that certain cultures could be considered more “civilized” than others is an idea born of the European Enlightenment, which, among many other reasons, developed the concept in order to justify the colonization of the rest of the globe. French, Spanish, and British colonists especially generated artificial hierarchies of “civilization” in order to convince themselves that the barbarous peoples of, say, South Africa or Southeast Asia were misguided and in need of European assistance. The idea that other peoples were barbarians or somehow less than human also legitimized imperial violence and the rise of the slave trade.
In direct answer to the question, yes, the Mongol Empire was more than barbaric and was much more complicated and dynamic than is typically thought. William of Rubruck’s account demonstrates this. For example, when William first makes contact with the court of the Khan, he and his entourage are treated with considerable dignity. Remember that it was William’s intention to convert the Mongol horde to Christianity, so the fact that he would even be granted an audience and a chance to make his case in the first place is a sign of the tolerance that the Mongols accorded to peoples of different faiths. Upon arrival, the group is offered either rice wine, mare’s milk, or honey mead as a refreshment. William’s interpreter, just as his party is planning on seeing the monk, informs him that the Khan had said the following:
Mangu Chan takes compassion on you and allows you to stay here for the space of two months: then the great cold will be over. And he informs you that ten days hence there is a goodly city called Caracarum. If you wish to go there, he will have you given all you may require; if, however, you wish to remain here, you may do so, and you shall have what you need. It will, however, be fatiguing for you to ride with the court.
Such appropriations are a clear sign of a welcoming host who takes especially serious the ethics of treating his guests with respect and dignity.
Another example of the tolerance (and hence, civility) of the court is demonstrated through the following quote:
The priests do not condemn any form of sorcery; for I saw there four swords half way out of their scabbards, one at the head of the lady's couch, another at the foot, and one of the other two on either side of the entry. I also saw there a silver chalice, of the kind we use, which had perhaps been stolen in some church in Hungary, and it was hung on the wall full of ashes, and on the ashes was a black stone; and these priests never teach that such things are evil. Even more, they themselves do and teach such things.
Keep in mind that William and his group likely condemned these sorts of practices themselves in the privacy of their own minds. Christian missionaries like him were not known for their willingness to countenance the heresy of non-Christian behaviors. But the priests and Khan himself are much more tolerant of differences of religious faith, a sign of both the expansiveness of their realm as well as their high mindedness.