Is there any specific reason to disable autocomplete?
if not, disabling that would add a 'little' dislike to your site.
Username should be enabled for autocompletion, Which is more true for mobile browsing, where typing is still quite challenging except the young users.
- Consider the major age category of your users. If that's over 25, typing on mobile is quite cumbersome. Add to that, someone may be using very small-display mobile.
- Usually, email-address is quite a long "word" to type in, where the mobile keyboard can't help with "auto-predict".
- A mobile is much more personal than a PC, so is its browser. So, filling up a username field wrongfully will be very rare.