Beth fydd y tywydd yfory? = "What will the weather be tomorrow?"
Sut bydd y tywydd yfory? = "What will the weather be like tomorrow?"
I'm not necessarily sure about grammatically incorrect, but it's certainly supremely awkward. I'd mark it incorrect on a fluency level.
"how will the weather be tomorrow?" makes perfect sense to me. I also thought "sut" meant "how"
As explained in the course notes, as well as meaning ‘How...?’, Sut ...? is also used for ‘What is/was.. something like?’