By Season 5, Don Johnson was participating less in the show as his goal was to build his movie career. He knew this was the last season. By doing that, it actually allowed the supporting characters to shine quite a bit more. Switek's gambling addiction, Castillo's Asian connection, and Tubb's The Cell Within episode (which featured a fine acting performance from Philip Michael Thomas. He was extremely underrated as an actor due to the series' hype). Sometimes Johnson would appear at the beginning of the episode and not be seen again.
But then there were those episodes that were all about Crockett such as this one. Most were great. And some were like this one. And what a miserable one at that. The only thing that gives this as high a rating as stated is solely due to Don Johnson's charisma. He is Sonny Crockett. The story with the cousin is reminiscent of every show looking to re-invent itself just before it gets cancelled. We have seen this on many great sitcoms (All in The Family, the Brady Bunch, Cosby) and dramas (Little House on the Prairie, Bonanza). The cousin, the brother, the new little kid, etc. They are brought in with the hopes of giving the central characters new energy. But, universally, each has always failed because that new character has always interfered with the chemistry and flow of that show's success. It takes the show to a new area that all of a sudden changes the show formula and thereby changing what the fans have always liked. It never works.