@Justin Xu,
The phrase "her head is ready to explode" is not to be taken literally.
Think of the annoyance she is feeling adding to her stress level and leading to a sensation of pressure building within her skull. The physical cause of the sensation probably has something to do with blood pressure, but the underlying source is her stress; her annoyance. If you imagine this sensation building and building; and without relief you can then imagine it becoming so great that that her head might be expected to explode.
The writer is not suggesting that Fey's head might actually explode, but that the way her head feels may be colorfully described as being ready to.
"Guilty pleasure" refers to enjoying something you think you should not, or which others would believe you would not based on the image you have projected of yourself.
In this case, I'm not sure the writer is using the phrase appropriately.
For watching Tina Fey get annoyed to be a "guilty pleasure," there would have to be reason to believe that watching anyone get annoyed was not something one would believe the writer would ever enjoy, or should ever enjoy.
It is also important that the "guilty pleasure" be genuine, that it flow from the content as intended by its creator, and not via mockery or lampoon.
For example, a conservative Republican pundit may say that he enjoys listening to the speeches of president Obama, because he enjoys making fun of them. In this case there is no reason for the conservative Republican to feel guilty about this enjoyment as it doesn't contradict any statement he has made previously: He thinks Obama is a fool and that his speeches are foolish. His enjoyment of the speeches are for these very reasons, and not because he secretly agrees with Obama or admires the man's eloquence. A writer would not properly use "guilty pleasure" in describing the conservative Republican's enjoyment.
However if a renowned Shakespearian expert and theater critic admits to watching and enjoying daily television "soap operas" because she gets caught up in the preposterous plots and stereotypical characters, this would be an example of a "guilty pleasure." Either implicitly or explicitly she has led the world to believe that she has disdain for soap operas and so there is reason for her to feel "guilty" about the pleasure with which they provide her.
"Guilty Pleasure" is a fairly complex and subtle concept with a number of conditions that need to be present for it to be used properly.
For instance, a "guilty pleasure" should not be used to describe an enjoyment that is deeply inappropriate.
A minister of a local church who is married with three children may, in fact, have a sexual obsession with child pornography, but a writer would not use "guilty pleasure" to describe it. Yes, it's something from which the minister derives pleasure and yes, he almost assuredly feels guilty about enjoying it, but the guilt is for a sin that is perceived to be much greater than violating the standards one has established for one's public face.
While it may not be absolute there is a general requirement that revelation of the "guilty pleasure" will result in mild embarrassment and not deep shame. Typically people have very little trouble with admitting to their "guilty pleasures" and are even somewhat proud of them.