It’s funny, the way that out of style clothes are perceived. You essentially have a 30 year cycle, where a style of clothing is fashionable for a decade and dated for two decades. “Mom jeans” are endlessly recycled, and kids do not have the same notions about how a mom dresses, unless you’re 30 years their senior.
Phrased another way, this joke is so dated that these jeans are actually cool again.