Suckville is considered by the Census Bureau to be part of the Detroit metropolitan statistical area, despite not being located anywhere near Detroit.
Megan and Cueball are playing a card game (possibly Dominion) and Megan whips a common insult, a play on the word "suck" that adds a typical city name suffix to the end of it. Other variations are: "Losercity", "Lameville", etc.
The phrase is originally based on the ubiquitous signage you see along American roads that say "Welcome to Town X — Population Y".
Then, since Cueball one-ups Megan by indicating there is a city by that name, she can only resign herself to the fact that her smack talk did not work. Since Cueball knows where Suckville can be found in the census data, it's likely that he has received this insult in the past (maybe even from Megan) and searched through the census data to prepare a comeback for this time.
The title text informs the reader that Suckville is classified as part of the Detroit area despite not being located there. Detroit, of course, is the quintessential example of a city so run down, it might as well really be named "Suckville", or include a district of that name. In fact, the city only recently (2013) declared itself bankrupt. (Fun fact: looking in the US Census Suckville is "corrected" to Saukville, Wisconsin, which is indeed not particularly close to Detroit, Michigan. However the population of Saukville in the 2010 census is given as 4451 for the village, and 1755 for the surrounding town municipality.)