Would having a healing mech Besides the lancaster be a bad thing?
The Balor already has exceptional healing capability applied strictly to itself, to the point that it fundamentally alters its relationship with Lancer's core gameplay loop. While it has a below-average Repair Cap, it fundamentally doesn't need as many Repairs as most mechs because it never needs to spend a Repair to come back to full HP after a fight, and it takes less structure damage because it constantly regenerates HP.
It would be bad if too many mechs had this power, both in that it would trivialize the Repair system and dilute the gameplay appeal of the Balor. But I don't think it would be bad if a couple of other mechs had somewhat similar abilities.
Another fundamental issue we've got to address when examining the Lancaster's healing ability is that the Lancaster's healing ability kind of... sucks ass? You take a quick action to make an attack that can miss and spend a Repair to give an ally half the HP they'd get from just spending a Repair themself - or one of yours, if they're standing next to you. There's a sorta-kinda action economy benefit if you hit (which, remember, is not guaranteed) and it has niche utility if your ally needs to pop off next round but is dangerously close to being Structured, but it's just... in most circumstances, your allies are better off just refilling their full HP during a Stabilize, which can also do other useful things.
Honestly the Lancaster's support traits are kinda mid in general. Combat Repair is "congratulations, you spent a full action and 4 Repairs to restore your dead ally to 1 Structure, 1 Stress, 1 HP, a state of being from which they can be sent back to dead with a single activation of an enemy with Reliable damage." Latch Drone, as mentioned above, sucks. The only genuinely useful support abilities are Redundant Systems, which is absolutely goated, and its ult, which it can usually only use once per mission.
All player mechs have the ability to heal themselves to full HP for one Repair and a full action. Any mech system or trait that heals HP (rather than some other mechanic like applying Overshield) needs to:
be better in terms of either cost, action economy, or both
not be so good it warps game balance