No in the myth she is never mentioned as one of goddesses or even as the goddess of faithfulness - one time she is even not mentioned as one of the high aesir while all other known ones are counted and described. In the EDDA she is only known as "Loki`s wife"
The thing with the faithfulness/fidelity comes from the fact that she still stays at her husband side and tries to safe him from pain while all the gods exiled and bounded him. Sigyn is not really a true mentioned goddess by the EDDA and norse mythology but a symbol for conjugal fidelity by people.
"I am Loki's goddess, la la la..." Oh, yeah! There's this book by Neil Gaiman; Odd and the Frost Giants. It's a story with Norse and all that! It's a kid's book, but it's really cute. There was this one part, Thor, Loki, and Odin are turned into animals and Odd find them. Thor's like, "When you turn into an animal, you slowly become wilder. The longer you stay as the animal, the more 'animal' you become. When Loki was a horse..." Loki interrupted "We don't talk about that." XD