Only if they are sick, and even then, they better have mono or some stuff or else no.
If they're really sick if they lip sync they could at least focus on keeping there energy up to excite the crowd. The point of a concert isn't just singing, it's also the hype, the excitement, and seeing your favorite artist bounce around on stage and talk directly to you, the audience.
I wouldn't pay for some stuff performance because the singer is sick, so she/he focuses on trying to keep their voice nice to no avail, and because of the focus on the singing the energy level of the performer is sub-par. forget that.