Is it true that Luffy's mother has been shown in the series? I've read some article that said Eiichiro Oda has drawn and shown 
Luffy's mother in the series. I forgot the source, but is that true or not?
We don't know. The only thing we know as Toshino mentioned in a comment is that Oda had said we would be very pleased with Volume 45 if we were wondering about his parents. Whether he was referring to his father Dragon alone or both his parents is unknown at this point.
From the SBS of Volume 44 chapter 

Reader: Oda-sensei!! I have a serious question. Will you ever draw Luffy's parents? P.N. Mr. Uni, Age 16
Oda: His parents? Well, I think you will be pleased with the next volume. Get excited!

Volume 45 consists of the chapters 431, Fist of Love through 440, First Fist vs Blackbeard. Luffy's father was revealed to the crew in chapter 432 and he was shown to the reader in chapter 440. Whether his mother was beside him in the base or somewhere else in those chapters is unconfirmed.