The last Marvel Snap game update sometimes forces the game to display in a narrow horizontal mobile-like window when it's not being played in Full-Screen Mode or with the window maximized. Under these circumstances, the in-game overlay has no room to display.
You can fix this by:
Playing on Full Screen or on a Maximized window
Enabling the Tracker Windows from the Untapped.gg Marvel Snap Companion, ticking the "Show Player Tracker Window" and "Show Opponnent Tracker Window". This will display the overlay in separate windows that will function regardless of the game's size.