While this article doesn't explicitly say why she is superpowered, it is explained in an interview with the co creator (Michael Green) as to why her role is so expanded in the series versus the book:
It was one of the earliest conversations Bryan and I had, sitting down to talk about our goals for a series adaptation: who gets expanded? And the first name on the top of that list was Laura Moon. She's very compelling in the book. Her scenes are extremely memorable and well-wrought. But she spends a lot of time off-screen and that seemed like the opportunity to deviate some.
Also, the strength of the actress (Emily Browning) chosen to play the role also plays a part in it:
Once we had Emily Browning saying that she wanted to inhabit the role, the idea of making her the third lead in the show, which we'd always imagined the character Laura to be, was very easy and was very comforting. Because we wanted more of Emily Browning
All excerpts taken from an interview with him, cited at Cinema Blend.
This is also corroborated by Neil Gaiman, as shown in this article, quote from Neil Gaiman:
Hopefully for Browning, “Git Gone” will strike a chord with viewers — at the very least, both Whittle and Gaiman loved it. “People say ‘which is your favorite episode?’ and I say ‘probably Episode 4,'” Gaiman said. “The joy of Episode 4, for me, is that it’s absolutely part of the ‘American Gods’ world and there’s almost not a single line that I wrote in the book in there. It’s just, you pull back and go ‘while this stuff was happening over here, this stuff is happening over here.'”
Also from Gaiman, he explains in this article that the book is written from Shadow's point of view, so Laura isn't in it as much and that he has thought about writing one from her point of view as well, but the series is the chance to strengthen her character:
One of the things they did get right? Expanding the show beyond Shadow’s journey. Laura Moon’s bulk-up is a standout, and for good reason. “She’s one of the most important people in the book. She’s just not in it very much because the book is from Shadow’s point of view. We can never know more than Shadow knows,” said Gaiman. “I occasionally toyed with the idea of writing American Gods that would have been Laura’s story.”
“Now we get that chance. American Gods is a TV show with lots of people and our camera no longer needs to be on Shadow all the time. Some of my favorite scenes are scenes that could not have existed in the book,” Gaiman continued. Creating something new is the driving force behind the Gaiman & Fuller collaboration, Gaiman especially delighted in seeing what Fuller brought to the fold."
So while not exact explanations, it appears that it is all with Gaiman's blessing and is nothing that he hasn't thought about before.
Edit: Also found this article, which claims it is because of Mad Sweeney's coin:
So long as it stays burrowed deep in her embalmed chest, the coin will continue to tie her spirit to her body, giving her superhuman strength — even as her flesh continues to rot.
This is also what is on the wikia site as well:
Object-based power: Mad Sweeney's "lucky" gold coin imbibes him with supernatural strength and unnatural prosperity. When he loses his coin to Shadow, Laura ends up with the coin's power, as seen in "Git Gone."
