It sounds like you're treating Luma as a direct attack method, when it should really be used as a method of stage control. The damage is nice, and some people don't seem to realize Luma Shot does damage at all until you hit them with it, but it's far from the main purpose.
If Rosalina is by herself, her only real kill moves are her smashes. Usmash is by far the strongest, while dsmash is actually fairly weak but can kill at moderately high percents if fresh and/or charged. Fsmash is in a similar boat, but less extreme. Luma's presence adds to their power and kill potential, of course, and turns fsmash in particular into another good kill move. Gimping is also an option, and Rosalina can do it fairly well since her recovery is pretty huge.
I've had good success by using Luma to keep the opponent busy, but not necessarily do a lot of damage by itself. When they make their way past it to me, I take advantage of Rosalina's long range to poke them away. With a little luck, they'll be in range for a followup poke from Luma, or perhaps some star bits. Rinse and repeat, recalling Luma and sending it back out as necessary to keep it useful. (If Luma isn't anywhere it can really have an impact, call it back. At the very least it'll add power to your pokes. Send it out again when it's safe.)
Against rushdown characters, use Luma as a rather durable but still expendable roadblock just outside your own attack range. Either they will try and attack it, which you can dash in and punish, or they will run past it and try to hit you, which you can punish using Luma. (Luma can attack regardless of what Rosalina is doing, including being grabbed or hit.) Aerial approaches can be met with utilt, uair, or fair. Nair is also an option but requires a good read since the hitbox starts behind Rosalina and takes a bit of time to move in front. When Luma is dead, make liberal use of grabs and take advantage of the absurd range on your tilts. Rosalina has disjoints all over the place, abuse them.
EDIT: When playing against a defensive projectile camper, remember that Gravitational Pull makes projectiles your female dog. Luma is also a good damage sponge that can take a hit for you when dashing in.
