But when one gets labeled as “serious” and the other doesn’t it most definitely IS a qualitative judgement. One is being dismissed as not serious.
On a functional level it’s just music. In the creation phase there’s really 3 methods: notated, un-notated but still pre-planned (which is most of rock music since almost none of them know how to notate and instead record demo tapes), and improvised (which usually involves some level of planning but it’s much looser). But really who cares how they got there. And really once you start categorizing you run into the kind of problems categorizers always run into, Frank Zappa wrote all of his music and demanded all his band members learn to read music, and yet he improvised constantly, so is his music notated or improvised? The correct answer is: who cares.
The important question isn’t how it was spawned but is it any good at being what it’s supposed to be. Trying to put artistic creations into boxes misses the point, and starts running into box limitations. Working on an SF convention we run into this a lot, especially lately as authors just don’t want to be in boxes and we start making new boxes like paranormal romance and urban fantasy. But we run into the same in music, all the punk acts worth a damn are heavily influences by Johnny Cash and Merle Haggard and other country acts. So does that mean punk is really country, or maybe Johnny Cash was making punk rock before there was even rock. Or you can just throw out the stupid boxes and enjoy the music.
No categories are 100% consistent but there has to be some definition that works most of the time. Notational music is the closest I’ve ever heard. If you play a Beethoven piano sonata faster than is indicated or play notes clipped where they were supposed to be held or change a harmony...then you are playing it wrong. In pop music these concerns don’t really exist. You can play a song any which way. The Serious-ness of it is that all aspects of a piece of music are important enough to be notated.
btw Improvisation has always been a part of the concert music tradition. Mozart and Beethoven improvised constantly and there are sections in concert works set aside for that purpose.