The vast majority of people love Jellico, it seems to come from a military worship background. “The chain of command can do no wrong” and other such nonsense.
If you believe Picard was right in Peak Performance that starfleet isn’t military, then it’s clear Jellico was terrible. If you come in to manage a team, you need to take them with you, and if you don’t, that’s your failure. People obey orders because they respect the person giving that order and/or because their position on the ship (and salary outside of Trek) and in society requires it.
if you believe Picard was lying and that starfleet is indeed a military, then maybe a commander who goes out of his way to stamp his feet and say “because I said so” is a shining example of leadership.
Necheyev made it quite clear that not only was the Federation not at war, that there was no one for it to become war.
Jellico comes in and like many terrible managers decides to change things without understanding what he’s doing, without saying anything to Riker about his concerns and soliciting advice on how best to meet his objectives. He was a paranoid war hawk eager to start a war so he could play general, but even then despite him preparing for war rather than fighting for peace, he still has time to worry about a fish in an office.
He was a poor choice to put in charge of the Enterprise
But let’s look at the other side. Say you agree with his hawkish tendencies (which Necheyev didn’t seem to), and believe that starfleet is a military organisation and believe that if someone comes in and says “jump” you should do it immediately, you have to agree that his order to divert crew effort from essential war preparations to interior decoration shows how awful he is.
He wa willing to completely drain his staff before any battle even started just in case there was a battle (engineering staff working around the clock for 2 days - What happened to that shift pattern - for marginal improvements in efficiency)
He wastes time he should be using for strategic planning and for getting riker on board (all it would take would be an accident caused by an ensign who’d been working for 48 hours to remove the fish causing a plasma explosion in the ready room and riker would be in charge) in crawling around Jeffries tubes, arranging sculptures, and worrying about his sons artistic work. If he were really preparing for war, he’s being terrible.
When Troi suggests appears he wants to talk about duty rosters. 45 seconds later he decides he doesn’t have time to talk about them.
Jellico has no concern about his crew, has no plan in case he becomes incapacitated, is determined to change everything about how the crew works on the eve of battle, including reporting the ship, destaffing engineering, diverting resources to decoration. He won’t listen to council from Picard either, instead attacking his morale, and confirming that unlike his superior he has already decided his orders will fail.
He was better in part 2, as we saw how wrong he was (a solution was found by the enterprise crew), but he still sacked Riker for obeying his standing orders of pointing out potential mistakes.
At best he’s doing what he can to start a war, but it seems to me that he’s a cardassian agent, doing everything he can to get the flagship of the enterprise working at minimum efficiency.