JoeGP | DeviantArt | website
Transcript
Comic in 5 panels
Panel 1: a guinea pig stands in front of a barber shop sign that reads “1/2 off Haircuts”
Guinea Pig: Oh, wow!
Panel 2: the guinea pig leaves the barber shop and sees his reflection in the window. He’s noticeable shorn evenly on only half his body from nose to tail.
Guinea Pig: Hey!
Panel 3: the guinea pig turns around and reenters the barber shop.
Panel 4: the guinea pig leaving the barber shop again spies his reflection in the window and sees a quarter of his body still has hair on it
Guinea Pig: What!
Panel 4: the Guinea pig re-enters the barber shop.
Guinea Pig: This is going to take FOREVER.


You say it’s not relevant to the joke, but a realistic reflection wouldn’t show a side-by-side depiction. The way it’s portrayed here highlights the hair cut that the animal is upset about, which is key to the joke.
It might not be realistic, but I see this as “artistic license,” along with the other absurd elements.
Since you can see what’s happened without the reflection, I don’t think a realistic reflection would hurt that prospect. In fact it might be easier (if the reflection were kept as it is, and the piggie rotated so that it would produce the reflection)