Have you ever noticed that other than Telephone rock all their music sounds like 1950's style rock? But yet, the characters are dressed like late 60's hippies. I find that pretty strange. After all, they sing one type of music, yet dress another.
Let us not forget, a relatively successful act during the 80s was The Stray Cats, who causes a resurgence in 50s rockabilly music... so, it's not entirely a stretch to imagine Gina being really into Little Jerry and The Monotones, also given that they're really big in-universe.Remember the 1988 episode where Little Jerry & the Monotones came to Sesame Street? You'd think Gina, who was 17 or 18 years old at the time, would be into an MTV-molded, snarling Billy Idol or scruffy Bon Jovi type. But no, the Monotones come strutting onto the street singing a doo-wop song called "Rock and Roll Star."
Interesting thing is, while Little Jerry's outfits changed throughout the years, I don't recall seeing Chrissy in an outfit other than his yellow shirt, except in the song "Raise Your Hand." But Chrissy's voice is so flexible…it's suitable for various subgenres of music…just compare "Count It Higher" and "Wet or Dry" (which I presume is the MJ parody you were talking about). There's only so much that Jerry Nelson and the Muppet songwriters could create, given that Little Jerry's singing voice was so high and, to be frank, a one-note kind of deal (no pun intended).A lot of young people do like older music. Especially if they've been huge, and their popularity was always consistent. Which I'll guess in-universe is what Little Jerry and Little Chrissy are. And on the show too. They've been playing Little Jerry songs up until the late 90's.
Of course, the complete opposite is true for Little Chrissy. His band looks exactly like what kind of music they'd play. Even though it's a caricature of Chris Cerf, he's obviously part parody of Little Richard (especially evident in earlier songs), I'd say with just a small touch of Randy Newman thrown in for good measure... I dunno... something about Exit reminds me of an older Randy style song. Then he sings a parody of a Michael Jackson song, and somehow, it fits perfectly well.