I don't think this sentence is a joke, it has a quite a mean feel to it. crap = shit, so if the bride heard this she would take offence and be very upset.

Yes. And it shouldn't be used anywhere that the bride or one of her close friends or family members could hear it. In fact, it shouldn't be used at all. But if you must, it is joking (or incredibly mean) and said far, far away from anyone at the wedding. The Bride is the most beautiful person at a wedding (the Groom is the most handsome). To say otherwise is a very, very, very rude.

That is a terrible insult. It is bad enough that it might result in the person's wedding invitation being cancelled if the bride heard about it. "Crap" is a vulgar expression; it means the same thing as "shit." The polite word is "feces," "excrement," or sometimes "stool." On a scale of 1 to 10, where 1 is polite language and 10 is the worst bad language, usually "crap" is about 2 and "shit" is about 6. "My car is an old piece of crap" is acceptable with friends. I'm told that a Canadian nickname for the Canadian Tire Company is "The Crappy Tire Company." However, to compare a _person_ to "crap" is always a serious insult. And a "bag" can mean a cheap, shabby, shapeless sack. A wedding dress might be the most expensive and beautiful dress a woman buys in her whole life. So it is a double insult, disrespectful to the bride's gown and to the bride herself. Now, if someone were making fun of a celebrity, a famous person on TV, she might say "that movie actress looks like a big white bag of crap." But to say it about someone you actually know is terrible. People who say rude or bullying things like to pretend that it is just a kind of joke. "Aw, I was just kidding. I meant no offense. Can't you take a joke?" But I don't think that any bride would _ever_ take that statement as a joke. Also, it could never be taken as a plain statement of fact because crap is rarely put into bags, and it would never be put into a beautiful white bag.

If you were using this expression in complete seriousness it would be very blunt and rude, but people would certainly understand what you are trying to say.