Not to mention, when they
did have real celebrity cameos they were often comical in their subtlety or execution, like getting Dustin Hoffman to play a substitute teacher and not making a bug fuss over it, or having Michael Jackson as a guest and crediting him as someone else...not to mention MJ didn't even play MJ in that episode...he played a nutcase at the mental hospital! Other times they'd bring in obscure celebrities like retired athletes such as Johnny Unitas or Joe Frasier, but they'd have like 1-2 lines of dialogue and it wasn't enough to disrupt the plot. The humor was that it was a famous person voicing a cartoon character in a comical, nonsequitir moment.
https://youtube.com/watch?v=OFV38GIaTSw
Hell, even the George Harrison appearance in the B Sharps episode was like this: he merely appears at the party where Homer ignores him to devour brownies, and at the end he drives past and scoffs at their impromptu rooftop performance. It was done subtlety and relevant to the plot, and not hamfisted like the Lisa Goes Gaga episode where the celebrity is the the entire focal point.
This is also what killed The Howard Stern Show when it moved to Satellite. The terrestrial radio show was fucking edgy and anti-PC, mainly because they had to strategically craft the show's dialogue to just barely skirt the FCC guidelines. They had edgy content, but their delivery to bypass guidelines was what made it funny. They would invite pornstars as guests, but since they couldn't words like "asshole", they'd refer to the girl's asshole as "fart box". Then once he moved to Sirius and he could use cuss words, the show lost its charm. It just devolved into a bunch of lame celebrity interviews with people like Ellen Degeneres or Amy Schumer. Stern's freedom to use cuss words doesn't mean a thing anymore: it's not edgy or shocking like it was in 1998 or so when he'd bring Jenna Jameson as a guest and ask her about orgasms on a national radio show.