YA SF is and has always been saleable. Heinlein made a great living at it, as did Asimov, Hamilton, and many others. There's nothing significant about time travel as opposed to space opera or any other theme. YA is YA because of the way it's written for a specific audience, so it can't be dark, have excessive erotica, etc etc.
But you do bring up a good question, maybe not in the way you intended. Some stories might work better as YA or adult, but that's based on the specific plotline and the author's style. So there isn't a general answer. You'll have to write it and see. You can always beta it and ask the reader if it might work better as one or the other.