I'm going to tell my sister that her wedding was cringe worthy
Why do you want to say that to your sister? Did she do something to upset you?
Because her dress was hideous, I've never seen something so ugly in my life.
Everyone is entitled to their own opinion so there's nothing wrong with not liking her dress, but there's no need to tell her that. You should instead be happy for her that she got married and had a wonderful day! Telling her that her wedding was cringeworthy would just upset her.
It's my sister, how I feel about her wedding is my business.
You're right that you are entitled to feel however you want about her wedding. Sharing those thoughts though will not help things and will just likely offend her. Don't you want your sister to be happy and have a good memory of her wedding? Keeping those thoughts about the wedding to yourself is probably for the best so your sister doesn't get upset.