One way to show that the light is inconsistent could be to have a series of panels specifically meant to show that? Like, wildly changing the lighting from panel to panel would just feel weird, but a series of, say, three similar panels of a character where the light and shadows on their face changes could get across that they don't have a stable light source.
But that's only if the "flickering" quality is super important, because something like that would kind of exaggerate the effect. If you just want the atmosphere that's caused by a single, unreliable light source, then focusing on the mood and colours of the scene and the way the shadows fall on the characters will do more to achieve that feeling than going out of your way to make sure we see the lamp flicker, imo. I think DownTheRabbithole's suggestions would also work for trying to get across the feel of a run-down place with bad lights.
My gut feeling is that if it's not important to the scene that the lamp flickers, then you don't need to spend extra time making darn sure that we see it flicker!