Whether you want to read for yourself or sing with your friends readability is pretty bad on the mobile app. You need to stick a phone on your face the read properly the small text floating from bottom to top. Also, changing the background color on the selected line is too much to digest for the eyes. There should be a subtle way for such a transition between the lines. I think it's not so much the point to create a big contrast as people follow the text anyway while listening to the music. It's more about reaching that text without too much effort for the eyes.
 

Here is one example:
I was driving a car and I put some music on the Apple Music app on my phone attached to the dashboard next to me. My friends on the back seat were able to read the lyrics without issue so they sing loudly all the way to our destination. On the way back I put my playlist on Deezer with lyrics and people started complaining about the font size so I needed to switch back to Apple Music where lyrics are carefully adjusted for close and distant reading. 

Not sure will that experience help someone in Deezer to rethink the concept of lyrics but I am sure many users would agree on that point.