The phrase "silly grin" refers to a smile that is funny or foolish-looking. It is often used to describe a cheeky or playful expression on someone's face. Full definition
There's a reason the folks who undertake «polar plunges» in winter months emerge from the frigid water with silly grins on their faces: giving your body a quick, cold shock stimulates deep breathing (which increases the oxygen in our bodies) and jump - starts circulation. (health.com)
That's about all the plot needed for this breezy comedy that triggers plenty of silly grins but rarely anything more hearty. (clclt.com)
Drive is soon shuffled to the front tyres to help pull you out, but not before a big silly grin has been plastered over your face. (autocar.co.uk)