Well a lot depends on what you mean by Roman (and boy is that a loaded conversation).
I haven't watched the series, but assuming it's an amalgamation of Viking fiction and history. Given that what I've heard of their sacking of a Christian monastery in England sounded a lot like the 793 sack of Lindisfarne, I'm going to assume that their raiding of Paris is either the 845 or the 885 siege, which would put them in the mid/late Carolingian era. I'm presuming the latter is more likely, given the drama, historical significance, and its rendering in other fiction like the Northlander comics.
So considering the "definition" problem, I see the following ways that we could construct Paris as "Roman", within the confines of this hybrid fictional/historical world.
Roman meaning Christian. In the Carolingian era, the franks had been in close communication with the Church and Bishop of Rome (the pope) in trying to "correct" the previously regional Christian practices into something more standardized and universal within the area under Carolingian authority as a means of enhancing their legitimacy. So with this in mind, Paris as a "Roman" city could mean Paris as a city which followed the Christianity of Papal Rome.
Roman meaning Frankish. The predominant "ethnicity" (equally loaded term) of Paris was those who embraced the culture descended from the germanic Franks. Gven the expansion of their empire under a series of strong "mayors" and kings under the Carolingians, ultimately leading to Charlemagne, they considered themselves the inheritors of all sorts of traditions, seeing themselves not only as the inheritors to Rome (a designation concrete by papal coronation of Charlemagne as "Roman emperor") but also as inheritors to the jews of israel as the new chosen people of the bible. So though to the Franks, being the "new Romans" was but just a small feather in their cap, I could see how you could construct the idea of raiding Paris, a Frankish city, as raiding a city of the Franks "who were the new Romans."
Roman meaning belonging to the geography of the former empire. There's obviously a lot of debate over what exactly was meant when Pope Leo crowned Charlemagne "Roman Emperor." Some of this is present in how Charlemagne chose to use the title, which because I don't have my books with me, I believe one translation was "Emperor governing the Roman Empire" rather than explicitly "Roman Emperor" so as to not be in direct competition with the Eastern Roman Emperor, who was still around. Which means one way to look at Charlemagne's emperorship, is him governing the geography of the former western Roman Empire. In which case, you can construct the argument that raiding Paris was raiding a city that was "still within the geography of the (former) western Roman Empire."
Roman meaning architecturally. I believe (but am not 100% certain) that even at this date, the palace at the tip of the Ile de Cite (where the Palace of Justice is now) was still the old Roman governor's palace built when the city was Lutetia, as in they hadn't torn it down and rebuilt it with newer medieval architecture. Also the same with the basilica on the island, if it was still standing, it would've been late Roman in design. The walls surrounding the island however, would definitely be Roman. So ignoring all other arguments, you could theoretically construct an argument saying Ragnar is raiding a "Roman" city on the basis of how it looks, namely late Roman in architecture. Keeping in mind though, that late Roman does not look like high Roman. It looks more early medieval, but obviously in the early medieval era, they weren't thinking themselves as such. A lot of older monumental architecture could feasibly be described as "Roman" to them.
However, if Ragnar means Roman city as in a city filled with classical style buildings, and people wearing togas, that would definitely NOT be true, in any possible semi-historical world and would be considered outlandish, as you just did which is why you asked the question.
I'm interested in seeing which scenario I proposed panned out (even if it's the last one). Get back to me once you watch it.