Is Luminel good in Lightning teams?

Her buffs reach Lightning allies, so every non-Lightning slot wastes part of her kit. Spotlight Moment raises the basic attack damage and BREAK efficiency of Lightning-type teammates (the game also calls them Electric-type) for 15 seconds. Her ultimate puts down a lightning arena that grants a 30% Lightning elemental damage boost. Put her next to one Lightning attacker and most of that does nothing. Fill the team with them and every buff lands on every damage dealer.

The usual spot for her is the DPS sub slot. She is there for the Lightning buff, not for her own damage.

Break and healing on top of the damage buff

Luminel also pushes break. Lightning Surge raises break efficiency and Lightning damage for the team, and her skill takes break efficiency to 24. Turbo boosts break as well, so you can pair the two if breaking enemies is slow for your team.

Her Prismana form gets a laser ability that turns electricity into healing light and restores allies' HP. That gives a Lightning team some sustain from the same slot that buffs it.

How to gear her

Her buffs scale with her attack stat, so give her attack gear even though she is a support. Defensive gear on her means smaller buffs for the whole team.

Not settled: The exact numbers on Spotlight Moment are not settled. The basic attack damage bonus appears as both 17% and 27%, and the BREAK efficiency bonus as both 10% and 16%.