Shraddha Kapoor is a mystery woman we don't know fully about but she was not Stree, she was after Stree's powers but we don't know what she will do with it. There is also the possibility of promised sequel to explore it:
The film’s director, Amar Kaushik, who spoke to The Telegraph on Sunday over the phone from Mumbai, said: “Right now we are busy deciding the details of the sequel after which work on the story will begin.”
Also from IN.com:
Raj Nidimoru says, “Stree, is an absurd horror story with a take on mob mentality, superstition, feminism etc so we kept it open-ended. But it picked up so much that everybody is demanding a sequel now. Fact is, we had a trilogy in mind when we started Stree as we know what’s going to happen, what happened before but we never thought we would follow it up so soon. We had a setting or an idea or concept in mind for all the three stories in the Stree trilogy – the spread of where stories like this can go and we toyed with it. A true sequel takes the lives of the characters forward. In some franchises, the concept drives the franchise and changes everything from the actors and characters to the story. In our case we want to retain all the characters, including Rajkummar and Shraddha.”
Also, for mystery girl he said:
“We may not answer all of them or immediately but the idea is to take it up from where we left off – maybe not exactly at Shraddha's bus ride but later,” adds Raj.
From filmcompanion, the producer answered it pretty well too:
What’s your explanation of the film’s end?
The film really ends with what Stree wanted and then giving her that. So we went from ‘O Stree kal aana’ to ‘O Stree raksha karna’. So they understand what she wants and that’s how the film ends. The explanation at this point shouldn’t be given. As a filmmaker because I would love it for people to interpret it themselves.
He even continued:
But what was the significance of Shraddha’s character stealing Stree’s hair? Are you hinting she could use that power wrongly?
She had come with an agenda and she got that. Her intentions seem noble and we hope and pray they are noble. The idea is to keep you intrigued so that we can explore something really cool in the second instalment. We haven’t even written it and I’m still in two minds over which way it could go. There are three options – I’m trying to figure what is more exciting. I have an idea but that’s the plot of the next one.