The fact that Sweden does not have a coronavirus hotspot as a neighbour further indicates that the Croatian outcomes have been much better.
More relevant measures would be things like urbanization, where Sweden is number 23 in the world and Croatia number 104 (by https://en.wikipedia.org/wiki/Urbanization_by_country). Or, perhaps, the population density of the most dense areas (5,200/km2 for Stockholm, 4,055/km2 for Zagreb). These are just examples of possible metrics, of course there are many more which might be more relevant, but population density of the entire country has no relevance at all.
Not really. Sweden has a low population density (23/km²) because most of the country has no one living there. The Stockholm region, where the spread of the virus has been the highest, has a population density of 5200/km².
(Numbers from wikipedia)