Well... Let's compare their stats.
Gardevoir has very good SpA and SpD, and a decent speed of 80, rest all are around 65 (which is bad)
Hatterene has her SpA stat a little more than Gardevoir, decent Def and SpD, but lacks in HP (so high defense is waste, but better than Gardevoir's low HP and Def) and horrible Speed.
Now let's look at the movesets.
Gardevoir gets access to various attacking moves such as T-Bolt, Shadow Ball, Energy Ball, Grass knot, Focus Blast etc along with Destiny Bond.
Hatterene has less wider movepool when compared to Gardevoir. The access to Trick room makes it to attack first most of the time.
The rest all are the same.
So overall, Gardevoir is good with your moveset given (But I recommend you to replace Calm Mind with Light Screen). Hatterene can play the role of anything you wish (Sweeper with Trick Room, or Special Tank or a Wall). In comparison, Hatterene is definitely better than Gardevoir.
Go with this moveset.
Hatterene @ Life Orb
EVs: 252 HP / 252 SpA / 4 SpD
Quiet Nature (+SpA, -Spe)
- Trick Room
- Dazzling Gleam
- Mystical Fire / Shadow Ball