Won't call myself an expert but from my own observation, even last year when Bitcoin was going strong, there were times when Ethereum was struggling a lot (the Crypto kitties episode for instance).

However in January 2018 it was the other way around, Bitcoin declining while Ethereum going strong, unfortunately for the latter it did not last too long.

If the correlation is less obvious than before, it is good news for several reasons: It means people buy the coins for the very concept behind the projects, and not only to speculate. Before the concepts of both coins are obviously very distinct, it makes sense that in the future, there will be less and less correlation.