Hello @COFIDENCE and welcome to the Community!

Third-party (custom) fonts may not render as expected on all devices and inbox clients due to configurations and limitations on the client side, rather than on Klaviyo’s side. If Comic Sans is rendering correctly in the Klaviyo template editor, that suggests that the discrepancy is caused on the inbox side. 

Here is a list of email clients that do support custom fonts:

• Apple Mail
• iOS Mail (the default email browser on iOS)
• Google Android (except Android 2.3, which doesn’t support the @import method used by Google and Adobe fonts)
• Samsung Mail (Android 8.0)
• Outlook for Mac
• Outlook App

Are you viewing the email via one of these inbox clients on your mobile device?

Here is another Community thread discussing a similar issue:

Additionally, here is the help center article on adding custom fonts plus some best practices:

How to add custom fonts in email templates (classic editor)

As suggested in this article, it’s considered best practice to select a fallback font that will appear if the original custom font isn’t supported on the client side, such as Arial, Georgia, Helvetica, Times New Roman, or Verdana.

I hope this helps, and thank you for using the Community!

-Brian