[[THIS SETTING DESCRIPTION IS OLD AND OUT OF DATE. SOME ASPECTS OF THE SETTING HAVE SOLIDIFIED DIFFERENTLY.]]
Being a magical girl makes you magical girl gay (attracted only to other magical girls); this is the minimum mental change that you get if you only make enough cosmetic changes to get magic at all.
You become more magically powerful (oomph, duration, do-what-I-mean, "luck", etc.) the more fabulous you look within the magical girl aesthetic. Even a basic-bitch magical girl package is happy to generate or doctor clothes, hairstyles, grooming adjustments, and accessories infinitely. You can also go back into the chargen screen and rearrange or add (or remove, but not drop below baseline) whenever you want.
Even a minimum girl can't pass for human. Even if you start out really ugly and want to change race, you're not going to get away with anything less than colorful gravity-defying anime hair, weirdly luminous skin, and pointy ears; if you start out pretty you need a nonhuman skin color with cute little tattoo-looking ornaments and fangs and maybe a few scales or feathers. It's very common to have wings, some people do tails. Adding more stuff doesn't help you except insofar as it makes you look more fabulous.
To be remotely competitive in a magical girl career of any kind you need to be wearing a fancy, themed outfit with a lot of Stuff on it - there are sharply diminishing returns after a point but you can't get away with a little black dress. Skirts typically work better than pants unless you really rock the pants. Fighting naked is better than fighting in stuff you found dumpster diving but it's not actually good. You get bonus temporary power for a cool transformation sequence from your civvies to your magical girl outfit, and it seems likely that people who present more fabulously (not necessarily to the central magical girl aesthetic) in their downtime may enjoy magic-related benefits. You do still have magic when you are not transformed but not much.
Powers will be on-theme and vary girl to girl how freeform vs. "spell-like" they are. Calling out your attacks helps but only if you name the attacks aesthetic things. You have access to more of them the cooler you look, know which are in your toolkit via slightly imperfect intuition (you're more likely to think you don't have something you do than that you do have something you don't), and need to keep on top of fixing your battle damage to your outfit during long fights.
You get access to chargen anywhere between ages 8-16. If you ignore it, it will go away within about a week. If you use it for under-baseline changes, it will take longer to go away, more like a month, though you get to keep your changes. If you get up to baseline, your magic and the chargen associated sticks around till you lose your powers.
Mental changes as you take more adjustments are mostly being more xenophilic, including liking the sorts of things that look like magical girl cosmetic alterations; being more interested in fighting the monsters; being more attached to staying a magical girl; being more open to/interested in polyamory; and generally Rolling With Weird Stuff. If you make truly profound changes (suppose you turn into a nudibranch), then there's an abrupt psychological cliff; magical girls who cross it (it's not too far past "furry") are clearly still intelligent and react in some ways differently to things they were familiar with before, but are no longer psychologically human or themselves and run around doing inscrutable things instead, sometimes coordinating (though not in any human language). Let us temporarily term these girls "cryptids", though they are generally understood to exist. Cryptids are unreliably helpful against enemies.
In addition to aesthetic-related and cosmetic-alteration-related boosts, magical girls get occasional sudden bursts of extra power when they are protecting/avenging loved ones (especially fellow magical girls), make any sort of successful communicative effort with any cryptid (synergy bonus if it is a cryptid of a loved one), or sincerely, seriously considering giving up their powers for any reason.
You lose fabulousness points for plagiarism, but if you lose any for having outside help with your costume it's not enough to detect under controlled conditions.
About 2% of girls get chargen as an option.