So yes Diana can see Ria's true appearance because she is the reincarnation of previous Saintess and her powers allow her to see real appearance of Ria. Actually when Ria woke up, she saw that she was wearing a necklace and female lead thought that this necklace belongs to her real family. Because of that, she always kept the necklace with her. So Ria figured that Duchess is her mother and told to the Duchess that she is her daughter, but when she said that duchess got really angry and say bad things.
Then Diana came and tell everyone that mc has beautiful pink hair and pink eyes, and she started crying. After that, they went to the temple and ask for a bishop, and when the bishop came, he used his divine power and reveals mc’s real look for a moment. it was really short, because the power in the necklace, which is too powerful, was more stronger.
And it happened in Chapter 123 of novel