Not to nitpick, but I don't think the demographics/tension were really related to Mariel.
McDuffie was murdered in December 1979. The first acquittal was on May 8, 1980. The remaining officers were acquitted on May 17, 1980. The riots began that evening.
The first Marielitos arrived April 21, 1980, but only about 7,000 arrived in April (out of 125,000 total).
After the riots were over, I believe that tensions increased, because the City of Miami (and Miami-Dade Metro) began to focus their attention on the Marielitos over the concerns of the black community (this was their belief, I'm not taking any particular side).
But the tension that cause the riots had to do with the relationship between the black community and the white majority, the Cuban population really didn't factor in to that riot.
Now, by 1989...yes. By then, the black community had spent a decade observing how the Cuban community was able to gain power and influence that had long been denied to them.
On a personal note, in 1989, myself and some of my fraternity brothers were in North Miami being trained to sell t-shirts/merch for Super Bowl XXIII, and I was driving down I-95 just as the riots started. This was pre-cell-phone/pre-Google-Maps, and fortunately I knew my way around Miami, so I got off of I-95 and drove west until I hit Red Road, which I figured would be far enough west. A couple of days later, we were doing a fraternity "**** week" thing over by the old Faculty Club, and the next thing we knew we were surrounded by a couple of dozen Gables/South Miami cops who thought we were going to riot. Good times.