It depends what climate you're used to where you live. Temperatures start dropping in October and range around 21-27 degrees. By HK standards, that is cool so most of the locals will have abandoned shorts and be covered up due to the "colder" weather. Certainly at 25 degrees, I usually think "ooh, better wear a cardigan". So in that sense, you may look a little odd but there are always lots of tourists milling around and I'm sure some will be wearing shorts.

Suggest you bring an alternative to shorts as HKers do like to dress up in the evening so depending on where you are, you may stand out a little if you're wearing shorts.