No. The horse-riding lady is there to spark tension and conflict in the story. The real drama here is between Jacob and August.
It's not a love story, unless you cast it as a love story between the author and the topic.
As a period circus drama, it stands well beside other classics of the (limited) genre, such as Gary Jennings' Spangle.
Now, I can see how one might mistake it for a love story. The two parties meet on almost the first page of the book, they overcome immense difficulties to be together, and at the end, they are together. Kind of.
But No. If it were a love story, then it would be told by the lady. She is married, she has her horses, she has a comfortable life. If she chooses Jacob, she gives it all away. Obviously the tension and conflict in her mind is a great story.
But she's drawn as a bland and boring character. She doesn't contribute a real lot to the story.
Furthermore, if it were a love story, when Jacob has a chance to leave with Marlena, he would take it. He's got Camel and Walter to worry about. If his first priority was Marlena, there would be no question about his decision.
Sara Gruen is telling the story of the circus. Backstage, mostly. Very little of the events occur ringside. The cruelty, the violence, the processes of the circus are what the story is about.
And there is where you see the elements of the true love story. Right on the first page we meet the circus. Backstage. There are difficulties along the way, the characters of the two parties are drawn with considerable care, and they end up together on the final page. For real.