Regarding the "preachy", "commentary", and "only pacifist is worth while" stuff.... I wouldn't say the game is TOO harsh on you, at least not until the very end. Most of the time the monsters are conveniently absent or not monitoring you when you kill their friends, so most of the time you're treated nicely regardless of your kills. There are of course a lot of times where kills are referenced but it's not a constant "you're an evil psychopath!" Or anything, most of them recognize that in some cases you had no choice and had to defend yourself.
You are "judged" at one point near the end, but it kind of just asks you to draw your own conclusions and reflect for a second. There is also a someone who hints to you how to work towards the pacifist ending, but if you kill that person the game doesn't even bug you to replay it again and be more of a pacifist and really... most people will probably want to kill that person. Lastly there are over a dozen different "neutral endings" with a wide variety of results and happy/sadness, so with all that effort put into the neutral endings I wouldn't say that pacifist is the sole focus, it's more just an achievement... Like how in other games where there's 100% completions.
For me personally I'm really bored of games making me kill everything all the time like that's the solution to all the worlds problems, so I was more than happy to be a pacifist. Of course it helps that the mercy system turns every single battle into a fun social puzzle analyzing the character and figuring out how to pacify them, and the encounter rate really helps here.... There's really not a lot of encounters, instead of 1 encounter every 5 steps like in some games there's more like 1 encounter every other ROOM so you can explore around and have fun without constantly being bogged down by random encounters, so each monster feels like a real individual that stands out from the rest.
EDIT: actually, you know... With all this talk of JRPG classification, and not killing things I realized what Undertale really reminds me of: WRPGs! With all the choices and consequences, recognition of character actions, weird comedic characters, etc. I mean Mass Effect, Alpha Protocol, Fallout New Vegas, etc. treat pacifism much the same way that Undertale does. " Hmm, I can kill this guy or spare him, hmmm... I'll spare him so that I can get more content from him later on." I mean that is how it works most of the time in those games; it's just that with Undertale instead of having the option to spare a few main characters you have the option to spare everyone.