What does “Swish of skirt” mean? Why is it a sexist expression?

'Skirt' is not sexist by itself (where 'sexist' means derogatory by sexual connotation). The word 'skirt' simply refers to a short dress, a garment these days that is worn mostly by women.

'Swish of her skirt' is not sexist by itself, where 'swish' ostensibly means a swaying back and forth movement. But the two words together evoke a bit of coquettish imagery, that the wearer might intentionally be moving her skirt provocatively.

A 'venomous swish of her skirt' is definitely sexist as it implies metaphorically that the 'swish of her skirt' (with the coquettish interpretation, is poisonous and therefore dangerous because of its female attributes, implying an out of the ordinary danger or tendentious difficulty based exclusively on the one-sided sexual characteristics of being female.

The phrase 'swish of her skirt' is not an idiom, but it is metaphorical. With 'venomous' it becomes very negatively intentioned.

So the phrase altogether is very sexist.

"Swish of her skirt" means exactly what you think it does. The phrase "venomous swish" has been parenthesised as it is a quote.

The use of "venomous swish of her skirt" is certainly sexist as the North Koreans are derisively drawing attention to the South Korean PM's gender rather than countering anything that she has said. While I am unaware of the full context of the dialogue, this could also be classified as an ad hominem attack.

To swish one's skirt at someone means to act seductively, especially if it is in a deliberate, calculated manner. (It does not necessarily carry negative connotations - we might admire a woman who swishes her skirt at a wealthy man to raise money for a noble cause.)

In the context, the “‘venomous swish’ of her skirt” suggests a dangerous harlot who has seduced a nation. It is an insult to her, as a politician, and to the South Koreans who elected her.

The fact that such an insult could only be directed against a woman makes it inherently sexist, although I doubt the North Korean political class really cares about such niceties.