If you are having issues with your email signature format on your iPhone, here are a few options to try out:
Option1: On your phone, go to Settings>Mail, Contacts, Calendars & scroll down to the section under Mail. There is an option titled Signature. Click on it & it will send you to a screen that gives you two options: All accounts or per account. Click per account & try copying & pasting the signature there.
Option2: If that doesn't work, try shaking your phone when you paste the signature. Apple tries to enhance the signature automatically, so by shaking it, it should undo the changes.
Option3: Try copying & pasting your signature from a forwarded email.