My decal set has no adults, just a sea of hundreds of the little girl figures closing in around a single cat.
There exists a current fashion among car owners to place decals on their back window that represent their family. The decals consist of stick figures to depict the parents and children, perhaps shown doing a favorite activity, and even pets.
The first car window features a couple with three children, while the other shows just a couple (Cueball and Megan), with piles of dollar bills and two large bags with dollar signs on them. The humor comes from the opportunity cost implied in this — not having children allows you to avoid the expense of raising them and accumulate money for your own use.
One might expect that the cars would represent the difference in wealth, and they are identified as 'urban SUV' and 'sporty hatch back' in the official transcript. The larger car is a Subaru Outback which is a typical car used by families. The second car is a Honda Fit, which is a budget compact hatchback, in the comic it has a spoiler added. The Subaru Outback is more expensive now than the Honda Fit, which seems to fit perfectly with the comic's implication since a family of five have to buy the large expensive hatchback. Being able to buy a smaller car that doesn't need to hold a five member family also allows you to save more money.
The title text refers to the humorous description of cats as the real masters of their household, and the little girls surrounding the cat refers to their ability to influence humans with their cuteness (as referenced in 231: Cat Proximity). One implication might be that any adults in the household act childishly around the cat, so they are best depicted as children. The title text could also be a reverse of the stereotypical "crazy cat lady". Instead of someone owning a very large quantity of cats it could be one cat with an ungodly number of little girls.