It's not your imagination or nostalgia. Modern pop music really is bad and it's getting worse every year. There are many reasons for this. (Sorry for the long tangent, this is a pet peeve of mine.)
Timbre is the texture, color, and quality of a song, its richness and depth. Timbral variety peaked in the 1960s and has been steadily declining ever since.
Modern pop music uses the same bland combination of keyboard, drum machine, sampler, and computer software. That eliminates the creativity of music and makes all the songs sound similar.
Modern pop songs also repeat a fifth-third-fifth note sequence accompanied by the vocalization: wah-oh wah-oh. It's so commonly used across many different songs that it has a name: "the millennial whoop." It's familiar and it sells, so every major pop star today uses this sound and it's becoming even more common.
The vast majority of the chart-topping music in the past twenty years was written by just two people. Yes, you read that right. Nearly all of the hits in the past twenty years were written by a Swedish man named Max Martin or an American named Lukasz Gottwald. These two men are the hidden song factory behind virtually every single band that is played on the radio today. That's another reason modern pop music all sounds the same.
Lyrics are shorter and repeat the same words more often. They've lost their poetry and depth.
The "hook," the part of the song that grabs you and pulls you in, happens sooner and more often in pop music today because peoples' attention spans have shortened. If a song doesn't grab you, you move on to another. That's one of the side effects of having millions of songs immediately available at your fingertips. Songs have to have more hooks more often to keep your attention. It's another reason modern songs all sound similar.
Another factor in modern songs is dynamic range compression. This process boosts the quietest parts of the song so they match the loudest parts. This effect is it makes the entire song LOUD. Producers do this to compete with all the other similar-sounding pop songs. They want their songs to be LOUD so they can be heard above the competition. This strips the song of its timbral variety and lowers the song's overall sound quality. Every sound on the track sounds the same. Again, this is why modern pop songs all sound similar.
Breaking in a new artist onto the music scene has become extraordinarily expensive. Production companies reacted to this by essentially forcing new artists on us. The good artists are no longer selected organically by the listening public. That takes too long and is too financially risky. Instead, production companies decide the winners by flooding the market with specific songs and artists. The public "decides" they like a new song because they hear it everywhere and figure it must be "good" because it's so "popular." This is called "the exposure effect." But it's all completely contrived and fake. It's essentially a form of brainwashing. We don't get a choice anymore.
Here's a 20-minute video explains all of this in more detail, for anyone who's interested.