Locust has really taken over the balance discussion lately. However, a common sentiment I hear is that Mesa is fairly underwhelming, and I agree.
Torso
Issue: Ability is not that impactful.
Harmonizer (the Core Gear) is probably the most disappointing bit about Mesa, it doesn't feel like it's doing a whole lot shield-wise. Also, the debuff is great, but the zone is so tiny that it's hard to utilize effectively.
I'd like to see the zone be made a bit bigger, boost the shield restoration a bit, and perhaps let Mesa gain the full shield value only if an ally is nearby (somewhat similar to Ceres).
Shoulders
Issue: Stats don't feel worth the weight cost.
We have one other 14 weight shoulder, the Bulwark (also Common), though that shoulder has to pay the 'Heavy slot tax' "(TL;DR: if it had a Light slot it would be 13 weight)*.
Even with that 'tax' they have fairly comparable stat pools.
Mesa has 28,700 less armor ( nothing to sneeze at), but 6,300 more shield (and Regen), and a faster recharge delay.
Those Mesa stats would be fine... if it was 13 weight, but as is it's a hard sell for 14 weight.
I would personally like to see something like this, buffing it while keeping its identity as the shield tank.
(LVL 13 stats only. Original -> Buffed)
Armor: 158,000 -> 158,000
Shield: 21,100 -> 24,150
Shield Regen: 21,100 -> 24,150 (because Defender)
Shield Delay: 13% -> 16%
Legs
Bulwark is the closest comparable set of legs.
Mesa legs have less budget, 1e/1w less, and then has a huge 42,580 less armor, while it does have a bit better mobility, I feel it could do with a very minor buff.