This happens a lot. Shorts are meant to be short of course, but sometimes they are just too short! That is what happened to my client when she bought some shorts for her twin daughters. The other issue is saving time and therefore money for the client. Rather than unpick the hem, I decided to just stitch the ribbon on underneath.
Doing the extension on shorts with blanket ribbon was a lot easier than using standard ribbon.
When she showed me the problem, I went through my ribbon stash and found some black blanket ribbon. I thought it turned out really well. Keeping in mind that the shorts are stretch and the ribbon isn't. But when they were on, the slight waffle in the ribbon disappeared.
Happy altering
Judith