You mean the Almond Eye comment? Well, in racehorsing Fillies and Mares are considered less desirable than Colts and Stallions because they tend to show lower performance and they are less versatile when it comes to breeding (A Male can breed hundreds of times in a year while a Female can only be pregnant once a year) so even if the performance gap isn't that far apart, you'd still choose to focus on a Male Horse because if it does become successful, it can leave a lot more stronger descendants.
The thing is that Horses have a lesser practical gap when it comes to physical differences between Male and Female compared to Humans (Strictly for racing)
The average Stallion can have a bigger muscular mass to the average Mare compared to the difference between the Average Man and Woman for example, but the physical differences specifically for Horse Racing are less practical, so Mares are sometimes capable of beating the most elite Stallions.
This is why Almond Eye's achievements are so impressive, she is the Japanese Horse that won the most G1 Turf races (which is the highest tier there is) and she was the Greatest Mare by FAR.
(I think she's the only Mare in the Top 10, but she's at the TOP)
I guess this also goes to show that Himawari is also the greatest