Sometimes a joke will depend upon an assertion that appears to be in stark contrast to, or even the opposite of, the truth.
In this case, although personal taste can vary widely, Kate Upton is considered 'very attractive' by the general American public.
Again although we all know that attractiveness is a subjective matter that can vary from beholder to beholder, Upton is very successful in her career, and her career is based solely upon her ability to be considered 'very attractive' to a general US audience. Therefore her career success is the evidence from we can surmise that the general US audience finds her 'very attractive.'
With that understanding, a poster says that Upton is a 6.
The number is understood to be a reference to a generally accepted US practice in which people rate the attractiveness of other people on a 10-point scale. This is really a widely understood rating system in the US.
So, on this scale, many people assume that 5, being the mathematical median, is an indication of 'typical' attractiveness. Obviously, the mathematical median is just a reflection of the scale and may not actually be 'typical.' The mode, the most frequently occurring rating in the scale, is actually a much better indication of 'typical' attractiveness; however, most people would consider the mid-point to be a good indication of 'typical' attractiveness.
So, the median is 5. Giving Upton a rating of 6 means she is only 10% more attractive than the median level of attractiveness. The poster is saying Upton is only 10% more attractive than the 'typical' person.
This assertion seems to be in direct contrast to the assumption that Upton is generally considered 'very attractive.'
So, this basic architecture of humor, once established, opens up to additional permutations.
It can be used to indicate that the entire rating scale has been skewed. If someone who is generally believed to be 'very attractive' is only 10% more attractive than the 'typical' person, then perhaps many other 'very' and 'extremely attractive' people are only barely more attractive than typical.
Perhaps, even on a 10-point scale, no one is rated higher than 6.
Also, it can be used in surprising, unexpected ways. Surprise can be a major component in humor.
Someone might indicate that someone who appears to clearly be less attractive than people would consider 'typical' is actually a 6.
Applying the 6 rating in an unexpected way becomes a humor permutation based on the previously established architecture of the joke.
Further, humor is also used as commentary. In this context, the use of the 6 rating might be a commentary about how the public's constant exposure to images of 'very attractive' people has desensitized us and made us jaded.
We have voyeuristic access to images of very attractive people 24 hours a day. Even though we might never encounter such people in a interpersonal activities, we take them for granted because the images are ubiquitous.
I don't use the joke myself, really. I don't find it very funny, but I hope that clarifies the 'why.'