Rather depends on the circumstances. The idea of a truly isolationist USA is interesting in many ways; not in the least because lots of people migrated to the USA in OTL, so the question becomes: where do they end up instead? But it matters when this isolationist policy is enacted, and what shape it takes. A USA that goes isolationist in the 1840s will be different from a USA that goes isolationist at the very end of the 19th century. (And yet different from a USA that goes isolationist in the 1930s, as has also been opted as an ATL from time tot time, but which lies outside the pre-1900 scope of this discussion.)
To answer the question very generally: the ATL culture wouldn't be similar to either of those two examples, really. Superficially, perhaps. But consider that both OTL's Britain and USA are nations that have historically been involved in world politics. That have constantly interacted with the outside, and that consider themselves representatives of... well, "the West". There are a lot of unspoken assumptions in the general worldview of the USA in OTL. In many ways, American policy has often been based on the idea that "peace and democracy" can be exported. That they are the logical outcome of history. Americans (not all, of course) often come across as if they just assume - entirely without being self-conscious about it - that their values are universal. Or that they should, at least, be universal. And that if exported, they will become universal.
An isolationist country, I suspect, will tend to think more on what it is. It will not consider itself universal, and will not wish to actively share its culture with the world. It will consider itself elevated; set apart. Its culture will not be something to be unselfconsciously strewn about, but something to be carefully guarded and cherished. What that culture gets to look like will depend on the exact POD, but here's a big indication: during all eras where isolationism was prominent in the USA, it was directed against the influences of Catholics, non-whites, Jews, Eastern Europeans and other 'poor' white immigrants (mostly meaning those from Southern Europe and Ireland, who were also seen as unworthy because they were typically Catholics). So an isolationist USA will, at most, let in (protestant) whites immigrating from Britain, Germany, the Netherlands and Scandinavia. The ATL culture of this USA could end up being rather... Germanic.
In any case, I'd expect there to be more of a tendency towards uniquely American culture (in art, architecture, etc.), and since the uniqueness of 'american culture' will more self-consciously be stressed... it will probably be more likely to be highbrow culture. No global 'pop culture' emanating from this USA!