I've eaten mutton in the US, in China, in India, in Afghanistan, in Pakistan, in Iran, in Iraq, in Turkey, in Greece… I think your theory doesn't stand up to scrutiny.

Edit: downvoted why?

I don't eat mutton regularly, but when I do I can notice the difference between mutton and lamb.

Last time I've eaten, like a month ago, we asked the waiter what was the difference between two plates and she told us basically that, one was more tender with a light flavour, the other one was stronger.

Just so you know. I did not downvote you.

India, Pakistan called goat meat as mutton.

Yes, still tastes bad to me (I admit this is subjective).