To clarify one other point, "Beaker" is one of the Muppets--the hapless lab assistant of Dr. Bunsen Honeydew. Beaker has a cylindrical head and does not speak, giving only wordless yelps of alarm at what Dr. Honeydew is doing. The viewer is saying the Oliver is like a living version of a silly puppet, except that Beaker is funny and Oliver is not. The viewer is also saying that although Oliver pretends to be a satirist and social critic, he actually "serves his corporate masters well," meaning the company that Oliver works for (HBO). The suggestion is that Oliver will never really "rock the boat" or say anything that makes HBO unhappy.

Yes, it refers to the bosses, but in a satirical way. In this context, "masters" refers to the master/slave relationship. So calling them "masters" implies that the speakers are slaves, and it is a comical/satirical criticism of the bosses.