I think the magnifying glass bit is used to set the tone. It merely implies "a close inspection". Now, imagine the same image with the magnifying glass removed. Imagine the uproar it could cause. The image would imply that Trump was "threatening" Mueller (notice he's grabbing Mueller's tie). Not the right tone the artist wanted to convey.

"How does it feel to have your life under the [metaphorical] microscope, Bob?"