I would certainly not say K has coached bad offenses. I think "ugly" is a reasonable opinion to have of his offenses in recent years, though. His offenses seem to have become more driven by helicopter parents of OADs, recruiting perception and weird, random loyalty to certain upperclassmen than by what would be most efficient for the team.
In 2015, he ran his offense through a player who is now considered 10-20 years out of his proper era in the NBA, OAD Jahlil Okafor, with something like 10x the post-up possession frequency of the best NBA offenses. Even more egregious, he was prepared to go the entire season playing Rasheed Sulaimon, with a 104 offensive rating, over Grayson Allen, who was a few months away from beginning one of the greatest offensive seasons in Duke history. He won the title game largely because Sulaimon and Okafor were unavailable to run his offense.
In 2014, OAD Jabari Parker had 32% usage. I could end it there, and that's insane enough for a program that's always loaded with talent. Parker did admirable work with such high usage, but his offensive rating was higher than only 3 players on that entire team. Those 3 Duke icons were Josh Hairston, Alex Murphy and a freshman Matt Jones.
In 2012, OAD Austin Rivers led the team in usage, admittedly at a non-atrocious 24%, but had the worst offensive rating on the team besides that guy whose place in Duke history is to make everyone else look better by comparison, Josh Hairston. Appropriately, Rivers' redeeming moment that season at UNC was a pure one-on-one iso possession with zero ball movement. This was the way K chose to go all season despite 5 other players on that team being good enough to earn NBA contracts, with 4 of them as juniors or seniors.
In 2017, it took 3-4 months for K to teach OAD Jayson Tatum what a good shot is and why that matters. Tatum's usage was hovering around 30% until K doing his job allowed everyone to see how good Tatum could be, for the final month or so of the season.
This hasn't been a pleasant stretch of Duke basketball to watch, and I think it's fair to question whether all of these offenses came anywhere close to reaching their potential.