I've been wondering this for a while, so I started experimenting. It has to be done from a desktop/laptop, and it's kinda wonky, but it works.
Open Chrome on a desktop/laptop. First make sure your Chrome desktop is signed into the Google account associated with your phone. Settings / Sign into Chrome. If you are already signed in you will see Signed in as 'gmail account' under settings.
Sort mobile bookmarks: Settings / Bookmarks / Bookmarks manager / Mobile bookmarks (left list) / Organize (tab) / Reorder by title. Done.
The wonky part is the syncing. The 1st couple of times I tried syncing my phone, it reset the desktop mobile bookmarks to sorted by date. I think it's best to just wait a few minutes after sorting on the desktop to allow the desktop to sync first. I don't see a way to force a sync on the desktop.
I'm not sure if this will "stick" or not, but I currently have my mobile bookmarks in alphabetical order on my phone, tablet, and desktop.