Yeah, linguistic difficulty is almost always relative - I can learn French or Dutch much more effortlessly than a native Japanese speaker. A native Korean (I'm guessing?) speaker would definitely have a leg up when learning Japanese that they wouldn't have with Mandarin, and that a native English speaker doesn't have with either.
The way I think of this for some Asian languages is that Japanese and Korean are like English and Dutch, while Mandarin is like one of the Romance languages (e.g., Mandarin is to Cantonese as Spanish is to French). The three have an easier time learning any of the other two for different reasons of shared vocabulary or grammar depending on the direction.
