Because of its smaller energy and the need for the KU Dest to switch roles between anti-capship and anti-fighter quite constantly, mortars are a bad choice. You'll do a little extra damage, but you'll end up being out of energy 80% of the time.
I've found my loadout I've used on my dest to work very well;
2 Infernos
2 Battlerazors
3 Solaris
2 Destroyer Missiles Turrets
I've been able to defeat a Liberty cruiser and a Raven's Talon alone with this setup, when any other cruiser I would fly would have been defeated (except maybe a Lib Cr of my own if I could hit the other one with my FWD gun).
When using this ship its not about trying to inflict the most damage possible in the shortest amount of time, but trying to take the least amount of damage throughout the entire fight. You can't stand toe-to-toe with ANY of the other cruisers and win except maybe the Bretonian Destroyer. Thats where the agility comes in. It can't outfly gunboats, but it can outfly any of the other cruisers by a small fraction. Your goal is to stay behind the enemy as much as you can. When they try engine killing to turn so you can't stay behind them, you inflict as much damage as you can before they can fire back. Good idea is to hit them with Solaris turrets when they can shoot back, and to dodge as much as possible during that small time. About only 20% of the other cruiser's weapons will hit you, depending on their skill, because the Solaris shake their screen. Also depends on your distance from them.
Continuing like that throughout the fight will let you inflict alot more damage than they do, which is crucial because the KU Dest has the smallest armor rating for any cruiser/dest other than the Talarca. When using this ship, its sometimes best to get enemy fighters first, as they are agile enough to shoot you all the time, whereas the enemy cruisers aren't. You gotta dodge the cruisers and sustain some damage, but target their weapons and blow them off with missile turrets and Solaris. Once the enemy fighters are no longer much of a threat (get most of their weapons, and missiles/mines are not a threat)
When you go to fight the cruisers, don't worry about shooting their guns off, just go for pure damage. Same for gunboats.
When fighting gunboats (Talarca and KU Dest are very good anti-gunboat cruisers) you should be hitting it with infernos and battlerazors from a very short range (Ku Dest can maneuver well enough to stay very close to a GB) and when the GB goes to hit you, use Solaris until its close enough to use battlerazors/infernos on it. There is also the option of using missile turrets to shoot the GB's weapons off (focus on Cerberus weapons) but it won't do much damage, and will take awhile. Don't use if you need the energy to shoot your infernos/battlerazors. Good for when the GB is at a distance.
Also, when dodging, DON'T use up/down strafe, but left/right as the Ku Dest is very slim, but not very short.
Make sure your Infernos/BRs are able to shoot forward, and your missile turrets should shoot forward as well. There is one turret that can't shoot forward if I remember correctly, and that one should be a Solaris turret (can shoot at an angle forward though, just not in the front 45 degrees, might be thinking of Ku GB).
If you DO use a Mortar, which I don't advise, replace the BRs with 1 more Inferno and 1 more Solaris turret.