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.