You've never met anyone who appears to be miserable and takes it out on everyone? I think that is one of the things that makes Minko such a realistic and strong character thus far. I've known at least a dozen people like that in my lifetime. It doesn't matter whether Ohana's actions were full of good intentions, everlasting love, or selfish desire to fit in. People like Minko as she is now just lash out anyway.
It's not a matter of whether or not Minko's reasons for being angry all the time are legitimate to you, they're legitimate grievances in Minko's mind, evidenced by her taking it out on Ohana in the first place. She's not going to take it out on Tohru because he is her superior. She takes it out on whomever she can.
The only way to fix the attitude of a person like Minko is precisely what Ohana is doing. You be nice to them anyway, over and over and over even if they don't deserve it. Ohana doesn't have that patience, and nearly getting into a car accident and realizing how scary it would be to die set her off, so she's going for her shortcut, which is to feed them food they hate.
I fail to see how this behavior makes Minko undeserving of any leeway over anyone else. It's not a matter of excuses one way or another. Much of Ohana's behavior is equally inexcusable. I find it disgusting that it would somehow be better if Minko was self-destructive in her loathing rather than the simple verbal abuse (and it barely qualifies as that) she's using. It makes it sound like you'd prefer it if she were nearly suicidal or some broken, pitiable KEY-series heroine.