In the early period, probably; for until 1102 they had their own, seperate kingdom, which came under personal union with the Kingdom of Hungary in 1102 because of marriage politics (the King of Hungary was heir to the last King of Croatia) typical of the age.

Because they were, thusly, dominated by Magyar (Hungarian) lords for most of M2TW's timespan, they were not created as a seperate faction, instead represented by a local rebel faction to make it clear that they were an independent, if not major faction.

As for going there... no, but I've heard it's quite beautiful, especially along the coast.