A member of the unofficial Mikata maintenance project was kind enough to send me a sample of some of the custom skins she can show up in, along with commentary that gives them a little context. Supposedly, there's more, but they tend to be a little wary about giving away 'her secrets'. Still, here's what I could get:

DEFAULT.pk
Default Mikata appearance.
COMMENT: "What's there to say? This is how she's looked for thirty years, and how she's going to look until we stop being able to get her to run on the latest version of the OS. Some looks never go out of style."

SUMMER.pk
Added in official patch 1.34. Appears when the outdoor temperature in the user's local area rises above 30C/86F. Requires internet connection.
COMMENT: "The site they were using for the weather data went out a few years ago, but there's a ton of weather sites nowadays, so it wasn't hard to get these to start showing up again. I'm really surprised that this is an official Mikata skin, given the appearance, but I doubt anyone was complaining."

WINTER.pk
Added in official patch 1.34. Appears when the outdoor temperature in the user's local area drops below 0C/32F. Requires internet connection.
COMMENT: "See above. If you're thinking of leaving a comment about the earmuffs, don't. We know she doesn't have ears, we know it doesn't make sense, and we know that you must've felt very smart when you worked all that out. You're really far from the first person to comment on it."

HAZ_MAT.pk
Bundled with the MKASystemSecure.pk content package. Appears when malware is detected on the user's system.
COMMENT: "This one didn't get much play, given System Secure's infamously poor performance as anti-malware. Mikata's a lot of fun, but probably not the one you want keeping your computer bug-free. Since System Secure is long-discontinued, we made this one toggleable in the Skins menu."

UNDERWEAR.pk (aka: CUTE.pk, VD.pk, 4U.pk)
Unknown origin. Sometimes appears on Valentine's Day (14/02) between 21:00 and 23:59, system time. Non-standard colour palette suggests unofficial origin. File name is not consistent across uploads.
COMMENT: "This one's weird. Adorable, yeah, but nobody from the maintenance project would greenlight something like this. I threw it through the decompiler and nothing about it is consistent with either the official stuff or ours. I figure it's some fan-made skin someone snuck in at some point that nobody wants to own up to having made."

DRESS.pk (aka: WEDDING.pk, SPECIAL.pk)
Unknown origin. Unknown trigger condition. Source file does not appear in Mikata.exe's installation directory, and does not appear in any official content pack, patch, or known unofficial modification. File name is not consistent across uploads.
COMMENT: "It looks like a hoax, right? Something some obsessed weirdo made. Maybe the most active Mikata users are obsessed weirdos, but why would they make something like this and then act like it just showed up in a random folder one day? Plus, what's with the folder they find it in always being passworded with their system username? Who made this?"