Thank you @Sharon Zhao-MSFT and @JamesTran-MSFT for your help and guidance. Here is the answer that I got. Hopefully there will be a feature to change the display name format for the Assignments soon.
"Teams assignment will always display names in the format lastname, firstname. Only way for you to display the name is different order is to change the first name to last and last name to first in the Azure Active directory. Now, we don't suggest this since all the MS app will get the user profile info from AAD, changing this will affect other apps.
We suggest voting for the feature request post here Option to display contacts by "Firstname Lastname" instead of "Lastname, Firstname" – Welcome to UserVoice!"