We don't know Brun and Renee's relationship, so it's possible that they are both comfortable with this kind of interaction. Maybe Renee doesn't really care, and Brun seems to always say whatever is on her mind. Renee possibly is used to it and wouldn't expect anything else.
But seriously? Being served food and calling it "dog vomit" is uncalled for. In fact, I'll go ahead and say it's a very, very dickish thing to say. "I'm just saying" has to be the lamest excuse for being insulting, too.
And I personally value honesty, even sometimes at the expense of being polite. I am not beyond criticising food that just doesn't taste/look that great, and even meals I enjoy, I point the flaws of, some of the time. I can veer into somewhat impolite territory with that, too.
But I would still not say unappetizing food "looks like dog vomit". There's being honest, and there's being rude. These overlap, but they are not the same thing, and confusing the two is not helpful for anyone.
Keep in mind, I don't think Brun is evil for saying what she said or anything. But I still think what she says is wrong (again, barring the possibility that it's just how she and Renee talk - some people casually insult their close friends as a weird show of affection that I don't claim to understand). It's insulting, it's thoughtless and it's pointless. It seems that for Brun, politeness and manners is something that happens to other people. Judging by what I've seen so far, she is unaware to an extent that a lot of what she says is improper and often very rude.
But being unaware that something is not OK does *not* make it automatically OK.
EDIT: I think Brun might be completely missing the point, saying "Probably tastes fine, I'll still eat it". If a guest described my cooking as "looking like dog vomit" and followed up with "I'll still eat it", I probably would not (because I'm big on hospitality and such), but I *would* be very tempted to take away their plate and being all "who said you still GET to eat my food? Buy your own damn food". I mean, seriously. Being offered food and snarking about it seems like a terribly rude thing to me.