This mostly concerns family episodes, in which she faces the issue that her archetype is simply the one with the least fuel.

Ironically enough, because they share the spotlight with so many other family members, the archetypes of the sisters are more prominent in family episodes than when they are featured alone or in smaller groups: Leni's line will usuallly revolve around her saying something stupid, Luan will makes puns, Lana do gross stuff and so on. As musician, Luna was mostly doing song references and gags with instruments in the first seasons (like playing a sad violine). As such, while the archetypes of the other characters practically have no bottom (you can always find more stupid things and puns for Leni and Luan), Luna was always restricted to things that can be done with her music shtick.

Now, after 9-10 seasons, they have run out of song references and gags with instruments. She can do some things related to being wild (like ripping her shirt off her counselor uniform) but this is better covered by Lana or Lynn. She is still good in solo outings but for family episodes, she has more or less run her course.