First off, let me state that Mirage is a very confusing boxer. Her style actually makes it so that she has a hard time against crappier boxers. If you look on the page you'll see that she has been knocked out by Korra and has been knocked out by Lois twice.
On the other side...
New in the league and still with an undefeated record, Helen had many reasons to trade fists with Mirage:
1. Helen had no problem knocking out Lois and Korra, two people who put smudges in Mirage's career.
2. Helen wanted to see revenge on Mirage because she recently fought her daughter, Violet Parr, and dismantled the young girl into the ground.
3. Helen wanted to teach Mirage a lesson for always flirting with Mr. Incredible, her husband.
However after all theses reasons, she couldn't seem to figure out how Mirage was slapping her silly each round. She couldn't teach Mirage a lesson to not mess with her husband. Instead, she turned into easy target.
In the picture above, she has gotten to the point where she can no longer accurately tell where mirage is.
A few clean punches subsequent to this picture, Helen throws in the towel and puts that first "L" in her career!