[mc32]

It's both a particle and a question mark word. [Ta]是外國人嗎?

This is how the question would be asked in the mainland or in the regional diaspora of Chinese speakers where foreigners are few. Where foreigner often is a substitute for the most prevalent non-regional foreigner (i.e. it's not typically used for Malaysian or Thai nationals in China) So for those who come over state-side they don't modify the phrase, they keep using foreigner [外國人] for any non-Asian, even when those "foreigners" are natural born.

[vidarh]

They clearly knew that, but was joking about the dual meaning of the question mark and mā as in 妈/mother, which is ambiguous when written out in an English comment where it's not a given why there isn't a tone mark (or whether or not they intent the English 'ma', for that matter).