The Utah Jazz would have done much better in the NBA Finals if they had not made the serious mistake of trading away Kawhi Leonard. Leonard had a key role in the team that they were not able to make up for with other players, and it helped cost them the Finals.
There is no denying that the Utah Jazz could have done better. However this is likely down to a number of issues as opposed to just one. It is easy after a change has occurred to then place the blame on the change as opposed to the current team. Saying that however, there is no way to deny that if Kawhi Leonard had not been traded, they could have done better. There are just many other factors which should not be overlooked alongside this.
It is too hard to play hindsight with the NBA. There is no way to predict if they would have done better in the finals with Kawhi Leonard. I don't think he was that much of a team player. It takes a team to work together in the end, and they were probably right that he did not complement the team.
Kawhi Leonard is obviously an great player, but his absence did not affect the Jazz's performance in the NBA Finals. A team is a group of players, not just one. It is impossible to know if he had been there if they would have done better, but the stress of the finals is the most likely reason for their ranking.