Should I replace the battery if my iPhone Max charge is 78%?

If referring to the battery health it’s because once it goes below 80% the degradation will be apparent (won’t hold a charge for as long for example). If you’re referring to your phone only charging to 78% then it would seem obvious that replacing the battery so you can get back to being able to fully charge it would be ideal assuming any other issues have been ruled out causing the issue.

Are you saying your Battery Health is showing 78% in Settings > Battery > Battery Health? Or you saying you can only charge your phone to a maximum of 78% charge?

If your battery health is indeed 78%, it's time to pay Apple to replace the battery. This link will help you schedule a battery replacement with Apple --> iPhone Battery Repair & Replacement - Apple Support