Putting a winky face at the end of something offensive doesn't make it a joke. As others said, if the group you're joking about finds it offensive, it is.

Honestly, I find the highlighted sentence, within the context of the paragraph provided very racist and offensive.

I understand you didn't mean any offence. I agree with others that have said pointing out the offensive portion and requiring you to edit it would have been a more fair way to go.