"Mocking an accent is a very serious matter as it ultimately undermines an individual's or a group of people's identity."
Undermines an individual’s identity. So the argument is not that mocking a person in general or making disparaging personal remarks during an intellectual debate is a bad rhetorical practice and an offence against the spirit of debate and is thus inadmissible, but that an individual has an "identity", which is a) sacrosanct, and b) can be somehow "undermined"; and that this undermining is therefore unacceptable.
It’s disappointing that the modern society prefers the latter argument to the former.
(BTW: I failed to recognize the word "matter" said in a Welsh accent; found the transcript on Google :-) )