Fired up sure I'll agree on that but that doesn't mean she wouldn't have taken the task as seriously as she did after all she was put in charge of cooking even though she was reluctant I'll bet that part of her is happy for the chance to serve her food and could be seen as training. As I said in my eyes she's full of pride when it comes to work and would have worked just as hard no matter if he was coming or not. Of course this is only a theory but I can't see Minko doing a half assed job in something when cooking is involved since it'd go against everything we've seen about her so far, can you see her doing that?
And again I don't think her romantic feelings to Tohru matters because when you've given such a chance like she was given thanks to him you'd want to show that they weren't wrong or at least show that you've improved, it doesn't have to be perfect like you said just not so-so. I would have done the same thing if I was confident that the omelet rice would only turn out meh.
The only thing I think she did wrong is the way she handled it but in her defense the other girl is the one that started throwing a hiss and scream, Minko on the other hand didn't reply angrily away but answered normally though her choice of words might not have been the best and then you got the girls ganging up on her immediately. Also we don't know what happened while Ohana was out so she might have explained why they can't make a good omelet rice on a hot plate.
Seeing as they're making such a big deal out of it I'm guessing it's not that easy, I've never made it myself nor cooked anything on a hot plate, the omelet might work fine but what about the rice etc? Then again you did say they could prepare it before hand. Of course if could be something unexpected like that fact that Minko just don't know how to make it.