Lesmok isnt viable atm.
With the extreme decrease in clip size you cant kill the enemy in one clip. Lesmok is currently just not viable for the mortar.
Lesmok Mortar can deal up to 1008 with one clip, not to mention it can always be buffed (and most of the time will be buffed). That's more than enough to kill most of the ships, especially considering that the DPS on lesmok rounds is higher than on burst rounds (262.08 DPS vs 209.92 DPS is quite significant).
To be honest efficiency of certain ammo types on mortars depends on how are you going to use it, and which ship are you going to fly with and against. Greased rounds provide highest DPS, guaranteeing a successful kill most of the time even if there's 2 engineers repairing the hull on enemy ship, but can be used effectively only on a very close range. Lesmok rounds on the other hand can shoot greater distances, which can get the enemy off-guard, and of course because of the increased projectile speed and lift they're also more accurate against ships with small hull hitbox, like squids or junkers. Burst rounds have the lowest DPS of all three, but can deal more total damage, so they are much more forgiving of missed shots compared to the other two, and their range is kind of a middle ground between greased and lesmok. Also, with one burst shot dealing 27 damage to the component it can be useful to slightly damage, or even disable enemy's guns, reducing their effectiveness by reducing their health.
Speaking of which, there's that one thing with burst rounds that kinda intruiges me, and since this is the perfect thread for it, I'll let myself ask you this. Artemis' AoE range is 3.5, 5.25 with burst rounds, and it is often mentioned that Burst rounds on artemis are effective because you can hit multiple components because of its wide AoE range. Now, with that being said, Mortar's AoE radius is 8 by base, so it already is significantly more than Artemis', and this AoE applies to Normal rounds, Greased rounds and Lesmok rounds alike. Why increase it to AoE radius of 12 with Burst rounds, giving you Lumberjack's AoE all the same, if according to the effectiveness of Artemis 5.25 is enough to deal damage to multiple components? Wouldn't it be better to rather stick to Greased rounds, significantly increasing your DPS while still having as much potential to kill Galleon/Goldfish, and still with the same disable power, or Lesmok, for increased DPS (less than greased, but still better than burst), but also greatly increased range and accuracy?