I doubt it. Visibility is not nothing, especially in other markets where tennis might be the #1 women's sports. Remember that tennis is a global sport, and these girls are traveling all the time. These lower-ranked girls might not be making millions off their clothing sponsorships, but depending on who they are, what their status is in their home country, what tennis' status is in that country, etc.; they for sure can leverage being ranked #100 for good money. And as you mentioned, it is so saturated that a brand needs to compete, too. If you're Adidas or Nike or even Lacoste or Asics, you don't want to lose out on presence.The tennis clothing market is saturated, it used to be 6 brands, now it's like 30. I'm guessing most players outside the top 10 not actually getting paid to wear kits, just getting free kits.
If Nike really wanted to move product, they would make more dresses that park players and juniors would wear.