Like most lines the dress code these days is rarely enforced. Carnival are very relaxed about the dress code & quite honestly I wonder why they even bother to publish one. On our last Carnival we had a family in baseball caps, shorts & flip flops on the next table, on Formal Night. We complained to the Maitre De to no avail. Carnival have 'dropped' Formal this year & now call it 'Elegant' night(jeans are accepted as 'Elegant' the mind boggles). To be fair though, Europeans do tend to make more effort at dinner, & I don't think you will have a problem with 'Smart Casual'.
I would not even entertain going on a Carnival cruise for the reasons that you have highlighted here. Spend your money elsewhere. What is the point of making the effort to dress up and look smart, when you have some idiot dressed like a beach bum sat next to you at what is ment to be a formal dinner? That is what happens when you lower your standards in an effort to maximise profits! Unfortunately it seems to be happening on most of the major cruise lines now. There will soon only be the 6* ship that have any standards left! Happy cruising.