Thanks

it happens in films, lyrics and newspapers. frequent grammatical error.

Did the movie actually use “saw”? Seen is correct. Or was the sentence incomplete, like “ Ugliest thing I’ve ever...” in which case “seen” was implied but not spoken aloud.

“This is the ugliest thing I’ve ever seen” is the standard (or “correct”) way of saying this. However, in some dialects, people use the past tense instead of the present perfect, or even a mixture of the two (as in this case). It’s something to be aware of when you’re practising listening, but don’t use it yourself. Hope that helps! 😊