Definitely a controversial subject that has been addressed by several paleontologist. The latest writing is a blog which Ive attached by noted paleo reconstruction specialist Scott Hartman. In his blog he addresses the case for and against lips. In his own word "I’m putting lips on all my non-beaked non-avian theropod skeletal reconstructions". More will be written but its an interesting read.
http://www.skeletaldrawing.com/home/the-lip-post1
Tom Carr's lipless Daspletosaurus
Article in Live Science that discusses Carr's no lips Daspletosaurus
https://www.livescience.com/58474-new-tyrannosaur-had-no-lips.html