Image URLs aren't MIME encoded in email
MimeMail is not MIME encoding url images in email

That's correct. Mime Mail is still in alpha and that feature isn't working correctly yet. If this feature is important to you please participate in the issue queue at https://www.drupal.org/project/mimemail
Images are usually only delivered as attachments if they are not publicly accessible on the server. A complete link to an external image should work just fine in any email client.
But let me also note that there is something going on that you didn't mention - the Mime Mail module will NEVER use 7-bit encoding, and will NEVER use quoted-printable encoding. So it's quite possible that you're doing something that breaks the behaviour of Mime Mail.