The issue when discussing art is its an entirely subjective thing.
Once you get past the "fundamentals" of something, it is all preference. And even then, some people can see breaking from the fundamentals purposefully as art itself.
There is something to be said with the notion of popularity defining what's "good". Take Mozart or Beethoven for example...are they "great" classical music because they are somehow significantly better by some measurable standard than other contemporary classical composers? Or did their music gain popularity that allowed it to live on through the ages better than other composers and thus the popularity is what essentially dictated they were "good"?
I think in terms of pop music...be it bubblegum, rock, rap, R&B, country, whatever pop you wish...popularity and longevity does say a lot about it. Is Adele a better singer than Britney Spears? Without question in my mind. However, I'd rate Britney as a greater and more influencial all around performer in a heart beat due to her longevity in the public, multitude of successful songs, and essentially helping to create the modern mold for the female young pop star as the first on since Madonna that truly captured such significant and lengthy attention.
In comparison to contemporaries in this age of music, I don't believe Michael Jackson is overrated. Indeed, I think some may underrate him due to his personal isses (Similar to how OJ Simpson is more apt to not appear on a list of the top 10 or 20 running backs in some peoples minds). Thriller was an absolute revolution with regards to music videos and how they were done. The man had a successful mainstream song in five seperate decades. Was regarded quickly as a prodigy as a young child and, unlike many child singers, managed to actually maintain his singing ability after being a child. He's got the largest selling record in the world and the most #1 singles by a male performer in the modern era. He's the only person in the Dance hall of fame whose dancing career focused around pop/rock and roll. He transcended music with regards to putting out mini-films, video games, commercials, etc. He became essentially a social icon as much as an actual performer. The glove, the moonwalk, the kick, the "cha'mone" line, etc. These are random cultural references that if you randomly did today most anyone above a certain age is going to INSTANTLY recognize.
As a musician, the guy was multi-talented in a package I would argue fewer have than the OP makes it out to be. His vocal abilities were excellent and are regarded by many critics as being a major talent. As I said earlier, he was thought of as a prodigy as a child. He had significant vocal range and stylistic abilities with his singing from your more fast paced rapping, to straight singing, hitting large highers or going with teeth clenching aggression. Also, not as commonly known from just his songs, he was rather adapt at beat boxing, mimicing various instruments and used that as part of his song creation method in nailing down tunes. Which leads to the next thing, unlike many current singers Jackson also wrote much of his own songs, which has led to him to be inducted into the songwriters hall of fame as well. His dancing skills were exceedingly good, and to my understanding he too generally was involved in the choreography of his videos. His stylistic range in the type of music he would work with was diverse, from motown to funk to more rock inspired beats.
In terms of the modern era, I'd definitely place him on similar footing as the Beatles, Elvis, and some of the other more highly regarded as extremely influencial acts of the past 50 years.
