I'm guessing himouto (干物妹) is some kind of pun on imouto since Umaru lives with her brother. But what's the joke? Googling the term in either kanji or romaji just returns results about the series.
himono literally translates to dried-fish, however the slang himono-onna is used to describe girls who put on a different face for when they're out of the house.
The top Urban Dictionary listing defines it as:
"himono-onna - girls in their twenties whom outside their home puts on a lively face, but as soon they get home, they change into their shirts and their hair are tied up like a samurai-style topknot, and they might even scratch their ass. snack in one hand, and canned beer in the other. on weekends they don’t go to blind dates, when they don’t have work they eat and sleep and eat and sleep and not a single guy around them." -- from Hotaru no Hikari (jdrama).
Apparently (as the UD listing says) the definition stems from the franchise Hotaru no Hikari.
As you can imagine, Hotaru no Hikari has a similar plot to Himouto Umaru Chan:
Hotaru works for a famous interior design company. It's a glamorous job but Hotaru's private life is totally the opposite of glamorous. She lives alone, and when she isn't working she's mostly lazing about her rented house in training wear. She's not interested in men. In fact, she isn't interested in anything. "I'd rather lay around than fool around," is her motto.