I'll have to go along with the idea of hating a character that you don't know anything about (those who have not read the comic), is just silly.
The Katana may seem to be an "unrealistic weapon" but only because you don't see or hear about people walking around with them all the time. Was it unrealistic back in the early days of this country for gentlemen to walk around carrying swords? What about during the Civil War? Oh it was war time, different circumstances.
It's war now with the dead.
It's not that hard to sharpen a sword, nor is it difficult to maintain. As long as the blade is kept clean after use, it's a low maintainence weapon. Far more so less than a gun which often times have to be taken apart to clean out gun-powder residue and other contaminants that'll mess up it's working and moving parts.
My character, John Handle uses a sword, a Katana like Michonne's... is he any less unrealistic? Sure he is... he's fictional, a figment of my fertile imagination. Rick, Lori, Daryl, T-Dog, Maggie, Glenn, Carl, Hershel, Carol, Andrea, Beth all of them fictional.
Granted I can see how folks were digging the idea of a Z-clypse and surviving day to day and learning what threats there are out there beyond the dead walking. Trust me folks, if it were to really happen, I'd have a katana strapped along my waist or my back where I can get to it and I'll use it. Hell, I'll carry one if we have a nuclear apocalypse (and I survive THAT). Silent and effective... just like Daryl's cross-bow, which if you think about it is a primitive weapon all in it's own right. Not to mention you don't have to worry about finding (or making) ammo with a sword.
It's still a mystery as to exactly why she has a couple of walkers chained and tagging along. Rendered basically ineffective with the loss of jaws and arms, I'd still consider them a threat irregardless. But without having to read the comic (only glanced at it now and again whenever I visit a book store) I suspect that...
Wait patiently for the start of the 3rd season and go a few episodes before passing judgement on Michonne.