FT, I agree with Lisa's response.
On Casual nights you are welcome to skip the the jacket, but on Informal nights, you should wear your jacket in and if you feel that uncomfortable with it during the dinner, you can hang it on the back of your chair. At least you made the effort. On formal nights, there is no question. Everyone was dressed to the max-- men were in tuxedos or dark suits, and ladies had their best evening wear on. That's the kind of atmosphere this cruise establishes from the moment you step aboard: you are greeted with a flute of champagne, not a glass of beer. : )
If you don't feel like dressing up to eat, there are many casual dining options. But then of course you would feel awkward walking around the ship after dinner for entertainment to shows, the casino, bars, etc... because everyone else will still be dressed up. It's the rules of the ship and while you may not be restrained to the brig by crewmen, other passengers may give you strange looks. You know, I was a little adverse to this too when I saw the policy but it turned out to be great fun. Give it a try. I hope you will enjoy it too!
PS: Be real honest with yourself, if all this feels like you have to convince yourself to go on this cruise line, it's probably not the right fit for you. Make sure you get what you want out of your hard-earned dollars and choose the best vacation for your needs.