Native English speakers are often able to go back and forth between various English accents with relative ease. This is often done in comedy.
Non-Native speakers usually can't do this.
What's the reason for this?
In the book "How Babies Think" the (English speaking) authors describe a linguistics experiment. They measured the ability of an infant to perceive different language sounds, including those sounds that were not part of their mother tongue. One example of the sounds was the "L" to "R" transition (a common stereotype of Japanese speakers). In fact, they played that transition sequence to some adult Japanese linguists, fully expecting them to hear the point at which the "L" sound became "R" and were intrigued when their Japanese counterparts didn’t hear the change. They found that, while very young babies responded to almost all the vocalisation changes of the world, between the ages of 6 and 12 months, they stop responding to changes that are irrelevant to their mother tongue. They physically can’t hear the difference between two different sounds that are equivalent in their mother tongue. And if they cannot hear the difference, it becomes much harder to mimic the difference (adult or child).
This is obviously a very small example and doesn’t account for so many factors, but given how complex accents are (sound formation, cadence, word choice), it might go some way to explaining not only why some actors can or cannot "do" specific accents, but also why different people perceive various accents as "good" or "bad".
Of course, in related languages, the phonemes are very similar, so it’s easier to mimic.
I appreciate that this might be off topic for this site as it’s not specifically "English" related, so quite willing to delete!