"imitation is the sincerest form of flattery" comes to mind, though I get it's hard to take it as a compliment when it's so commonplace for you.

That said, in that moment, they're not "having fun" (in the mocking sense) they're relishing the sound of your voice and wanting to imagine a version of ourselves being uniquely interesting, like you, in the mimicry.

I get that it gets old, though, as your accent isn't novel to you.