Look: the currently accepted scientific definition of "planet" excludes Pluto, but the scientific definition is a empirically-derived descriptive taxon and part of a system of empirically-derived descriptive taxa.
Modern Western thought's insistences aside, you are not personally beholden to scientific definition when speaking as a layperson. "The planets", in everyday thought, are a set of objects blessed only by heliocentric sense of wonder that they're worlds, as ours is. A set expanded by imaging technology, the pushing-back of the limits of sight, true, but basically a set of objects grouped by convention.
If a magical girl wants to call herself Pluto, the very fact of being a magical girl, rather than some kind of science tokusatsu, speaks to a connection to metaphysical, prerational, and/or transcendant cosmic knowledge, rather than a devotion to pedantic taxonometric accountancy.
In short, fuck the haters