Hi! Seems that ERPNext's hook variable default_mail_footer cannot be override by app hook's mail_footer:
default_mail_footer = """<div style="text-align: center;">
<a href="https://erpnext.com?source=via_email_footer" target="_blank" style="color: #8d99a6;">
Sent via ERPNext
</a>
</div>"""
I believe there should also be a global setup to use default ERPNext footer or your own customized.
So there is already a setting for this in System Setting:

Please close. Thanks!
Most helpful comment
So there is already a setting for this in System Setting: