If you were invited to a Teams meeting, or left early, and don't want to see the Chat from the meeting.

• Go to "Chat", select the chat stream,
• Click the ellipsis at the right of the chat name,
• Select "Mute"

This method can work for any chat... If you are in the middle of something and don't want the pop-ups, you can mute the chat until you are ready to read the chat. 

Another way to turn off Meeting Chat

• Click on the ellipsis next to your picture/initials
• click on Settings

• Click Notifications
• Under Meeting chat notification, select Mute