Lennon was an entertainer, and knew how to play an audience. He had little need or reason to live according to what he presented, and so he didn't.
The only difference between Lennon and Brown is that Browns private life is in the public eye, and one can no longer be an entertainer and have reasonable expectation of privacy, and behave badly without damaging their career.
Some types of damage are expected and acceptable, but abuse is not.
If Lennon lived at this time, he would be playing the same game. "I won't do it again, blah blah blah" in order to keep his career and image as clean as possible.
Another big difference is that Ono was pretty much an obsessed stalker, and he could beat her as much as he liked and she wouldn't leave him, nor make a big deal about it. The culture at the time supported some amount of what today we would term physical spouse or child abuse.
While he and brown are both obviously spoiled children, the circumstances are different enough that we can't say with surety that had Lennon lived now he would have done the same things as he did then, or that had brown lived then he would have gotten away with it as easily as Lennon had.
Regardless, they both suck as musicians. They had a few good pieces, but Lennon had more simply because he was more prolific and he launched off a successful band. But if you take a thousand pictures, the best few might compare favorably to an Ansel Adams print, and that's what we see of the Beatles and Lennon. They produced hundreds of songs, and today we know a handful of them well. Some of the rest are ok, and the remainder are junk.
The reason Lennon was successful is not due to his music, but his marketing.