It means something like this: "I didn't like that short girl you were dating. It's lucky she's so boring, because that means you'll stop paying attention to her, and start paying attention to Rebeca instead." The colloquial phrases being used here are NOT contemporary U.S. colloquial English. I've never heard them. Google tells me the passage is from Barbara Kingsolver's "The Lacuna," which takes place in Mexico City circa 1940, and it's a woman of forty speaking. The phrases sound old-fashioned to me. They could be 1920s slang. However, the meaning is clear enough--they're derogatory. The mother is trying to supervise her son's love life. She doesn't like the girl he took to the Posadas. She is glad that things are not working out between his son and the girl--"if the other one is a wet sock, that's your good luck"--because that means he might get interested in "this Rebeca." Aha. Wentworth and Flexner's "Dictionary of American Slang" says: "Wet sock. 1. A jerk. A dull, dreary person. 2. A limp, flaccid handshake." "Half portion" sounds like a reference to stature (as does "little jelly bean,") and Wentworth and Flexner say "Half-portion. An undersized person. c. 1925. Not common.") I don't think Barbara Kingsolver actually expects her readers to know these phrases. She expects them to get the meaning from context, and to get something about the woman's personality from the way in which she uses them.

I agree that wet sock basically means useless. It's possible that it might be used here in a sexual context... i.e. the girl is useless, so if she was "easy" (wet sock) that's your good luck.

Literally, a wet sock is not good for anything - you can't wear it, it is heavy, damp, cold, etc. So when you say something is a "wet sock", it means that it is "useless". In your example sentence and paragraph, I don't quite understand why the author writes "If the other one is a wet sock, that’s your good luck." I guess he means to say that the other girl is a wet sock (useless), but I don't know why that would be good luck. I guess I would need more context of the story. But anyway, "a wet sock" means "useless".

to describe someone or something as useless. Urban dictionary