Yes, user can change the custom labels on the form for success and error notifications by following these steps:

Step-1: Enable the “Translation Language Settings” form Setup to use the New Local Translations Option as shown in the image below -

Step-2: In the Quick Find Box, search for Custom Labels and click on it, and select the following custom label - UCD_Account_Form_Submitted as shown in the image below - 


Step-3: Now Click on the New Local Translations / Overrides button as shown in the image below - 

 


Step-4: A New Translation page is opened, where you need to choose the language and enter the Translation Text.


Step-5: Set the new label as you like and save it. The message will be changed to your new message on the form now.


Note: If you're using multi-language, then you need to create the new Translation under this label for different languages, which will then override the previous ones.