Yes, it is safe to use your smartphone in selfie mode to watch the eclipse, as per Jim Todd, who is the director of Space Science Education at Oregon Museum of Science and Industry. He told this in an interview in KOIN 6 News. In his own words:
"Selfie is fine. You are not looking at the sun in that fashion and you are taking a snapshot really."
But at the same time, you need to be careful about not to keep looking at the phone continuously throughout the eclipse.
As per the advice of NASA, "You have to be careful that you minimize glimpsing the bright sun with your eyes without the benefit of a proper filter,"
The other issue is, it is recommendable not to zoom in much while looking through the phone in a selfie mode, as the magnified view can affect the eyes as well as mar the sharpness of the snap if you take a few.