Myth: What makes music, art, or literature good is a matter of opinion.
What you like is a matter of opinion, but what denotes quality is not. Mankind has been making music for about as long as we've been sapient, and in that time, we've discovered a great many things that "work", and a great many that don't. We've developed a pretty well documented and understood specification of what constitutes music. This is not to say that an innovator won't come along and change those rules- think about the reactions the first time Jimi Hendrix broke out a fuzz box and hit the crowd with face melting distortion. That changed the rules.
There's an entire area of study called "Music Theory". My, isn't that a scientific sounding name? Let me tell you a little secret. It is science. Music Theory is a branch of applied mathematics, colored with some physics that can be used to quantify and analyze the structure, tonality, and other elements of music composition in a piece. That's the quantitative aspect. But based on those quantities, there is also a qualitative decision that can be made.
Some music puts the elements of theory together in an inventive, compelling, or dramatic way. This is "good" music. This requires originality; this requires talent. Other music lacks those elements; it becomes repetitive, homogenous, and corpselike.
Now, some people lament the state of music, but I must say- you're overstating your case. Yes, there's a lot more shitty music than there used to be. There's also a lot more music than there used to be. Every once in a while, a good artist opens up a new genre, pushing past a boundary. A short time later, a flood of people wanting to cash in on this virgin territory will follow. These people don't have the talent, foresight, or vision to originate, and so they imitate. Imitation is always a sign of bad music.
This is not to say that imitating someone else is bad, but Weird Al, who only imitates, adds in a layer of his own origination. Some, like PDQ Bach use unusual instrumentation. You can imitate and originate at the same time, but without that breath of creativity, without doing something that hasn't been done before and yet adheres to the established structures of music theory.
(That last bit there is important- originality without structure is no better than raw imitation. This is why I don't like noise bands. While I appreciate the innovation that they bring, to do it divorced of musicality is at best a gimmick.)