@Ichijoe  
It’s just a stylistic choice. While 私 (watashi) is taught at school later than 変 (hen) and 態 (tai), the former character is used so frequently there’s no way a native Japanese speaker wouldn’t learn it simply by observation. There’s no law saying which words should be written with kanji and which ones shouldn’t (if she wanted to she could have written kusomushi as 糞虫) and some people like to avoid “overloading” their sentences with kanji, or think that katakana looks cooler.
 
And no, the shi’s are clearly シ and not ミ, no matter how sloppy the handwriting is. The lower stroke is obviously not parallel to the other two and it’s also sloping upwards on the right, not downwards. The fact that the other two strokes aren’t sloping downwards might have thrown you off, but this is done very often in order to make the character more distinct from ツ (tsu).

Hello = i’m a pervert and shitmaggot
 
 
Well, looks legit for me

@Daruqe  
It sometimes boils down to personal preference, really. In my experience, most people use kanji, but some prefer to write in hiragana or even katakana.
 
It could also be showing her foreign origins, but I think preference is more likely.

Perhaps Twi is still a baby pony that hasn’t learnt enough kanji yet?
 
Wooh boy wont that be fun having to learn and memorize thousands of different Characters that not only represent phonetic sounds in and of themselves, but are words in their own right…

@Ichijoe  
Yeah, you were prolly confused because the way katakana シ was written here–sloppily. What I’d be confused about is why “watashi” is written in katakana here, and even I couldn’t give a coherent answer to that.

IT MEANS “I’M A PERVERTED DUNG BEETLE” NOW STOP ARGUING.

v Or actually just Buck…

Actually Google has クソ (ku su) down as bucking if ya know what I mean?

Or I’m into kinky sh!t?!

I’m kinky Kusomushi ??

@CamChao  
Beaten to it. :(

Well Hentai means more or less “Disgusting perv”.  
But then again rape is a legitimate way of saying hello in Japan.

@Ichijoe  
Three is san.

@Ichijoe  
Shi is four (ichi, ni, san, shi/yon, go). It’s four that’s sometimes pronounced “yon” rather than “shi”, because “shi” can also mean “death”. And in written kanji, three (“san”) is 三, again a distinct character from katakana “mi” (ミ - note the sloping strokes).

@Background Pony  
I know “kuso” is “shit” but the rest doesn’t make sense…

Is something like “i’ m a perv” and something right?

But can’t Three be also written / spoken as shi?  
I seem to recall having learnt it went like
 
Ichi  
Ni  
Shi  
Son (yon)  
Go  
etc…

A japanese askblog? Neat.

@Ichijoe  
It’s just the handwriting making them slightly ambiguous, when they’re actually distinct characters. In katakana, shi is シ, and mi is ミ.

This is what I find confusing about kana.  
Lets take Shi. Is this not the same Character as Mi? I really don’t want to get into why Four and Seven have multiple Names as well. But just in the case of Shi. I think it has something again to do with Death. al-la Shinigami.
 
So why can I read that Charater as both Shi and or as Mi as in Miku (ミク)?

A suitable phrase for any occasion.