When using support spirits that have magic attack up with Peach Bomber and her other ones there power doesn't increase like Zelda's Nayru's Love and other characters magic attacks. Does anyone know if this is a bug or intended?
It's definitely intentional, since in Smash 3DS / Wii U, the Peach Bomber is not classified as a magic-type attack; it's a body-type attack.
Peach only has two magic-type attacks, which are her u-tilt and u-air. As a result, Spirits that boost magic attacks aren't going to help her much.
And just to be clear, magic attack boosting (and resisting) Spirits will NOT affect magic
effect hitboxes; the hitbox
type has to be magic.