Why is it racist to mock someone's accent?

I am Northern Irish and I live in Greece and people mock my accent all the time. Why do people have to bring race into everything? If you mocked an Asian person's accent then it would be seen as racist. Why? Making fun of someone's accent has nothing to do with race as plenty of white people (who apparently can't receive racial abuse) have strange accents that people make fun of.
Why is it racist to mock someone's accent?
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