Some quick numbers:
Light Needler: 500 K burst, 340 seconds of ammo, 294 Dmg/sec vs. shields, 147 Dmg/sec vs. hull, and on average 11 Dmg/sec vs. armor. That's a flux ratio of 2.5 against shields, 1.25 against hull, and 0.1 against armor.
Light Lancer: 450 En burst, 562 seconds of ammo, 160 Dmg/sec vs. shields, 160 Dmg/sec vs. hull, and on average 27 Dmg/sec vs. armor. That's a flux ratio of 1.13 against shields, 1.13 against hull, and 0.19 against armor.
Light Needler is really excellent against shields, okay against hull, and trash against armor. Light Lancer is okay against shields, okay against hull (but slightly less efficient than the needler), and still not that good against armor. It may be more than double the armor damage up from a needler, but 27 Dmg/sec is still so low that it's barely worth mentioning. Let's look at another gun.
Light Assault Gun: 40 HE burst, 100 seconds of ammo, 80 Dmg/sec vs. shields, 160 Dmg/sec vs. hull, and on average 65 Dmg/sec vs. armor. That's a flux ratio of 0.5 against shields, 1 against hull, and 0.41 against armor.
Even a Light Mortar does more armor DPS than a Lancer. Simply put, the Lancer is not acceptable as an armor buster. You could make the argument that it's a general purpose weapon, but if you had the choice between two lancers (320 Dmg/sec vs. shields, 320 Dmg/sec vs. hull, and 54 Dmg/sec vs. armor) and a needler plus an assault gun (374 Dmg/sec vs. shields, 307 Dmg/sec vs. hull, and 76 Dmg/sec vs. armor) you would almost always pick the latter.
For the Lancer to remain competitive, it should have fewer shots that individually do higher damage. A half-sized burst with double damage (and flux) per shot should do the trick, bringing it up to an average of 34 Dmg/sec vs. armor. It would still do less damage than the needler-assault gun combination but have the advantage of faster shots and better range.
Fun fact: Aside from missiles and bombs, the Heavy Mauler is the best medium slot (or small slot) armor penetrator in the game. It comes just third when you include mods; only the Gale Cannon (by a small margin) and the Longinus Heavy Laser are higher.