Ion is still S-tier, she's just not as far ahead of the pack as she used to be.
The issue was that her DPS output was so high, that the intended trade-off between using your energy for attacking (laser-shot, alt-fire) or defending (vortex shield, trip-mine) rarely came to pass... most other Titans simply couldn't put out the DPS she could, at all the ranges she could.
You could always out DPS your troubles in any 1v1, and in most 2v1s.
Now you actually do have to watch your positioning and keep note of your energy pool, because essentially any use of defensive abilities will rob you of your laser shot for a bit of time. Zero-Point trip-wire is my new must-use over Grand Cannon now.
Still sad they nerfed her though, I'd been using her as a main since day 1.
Monarch is fun. I wish they let you save multiple configurations though. I have one glass-cannon config that's just maximum DPS, and another for max survivability. Pain in the ass to swap between them mid-match right now.