Ichika just officially put herself in the Worst Girl role... telling him dressed as Miku is bad enough... but saying "I'll be cheering for you" essentially trying to get rid of Miku's chance makes it even worse. Well the manga was fun while everyone was on a more or less equal ground, now Ichika just wanted to be last so bad.
This is exactly why Itsuki is going to win. The other four girls are going to start fighting to gain Fuutaro's attention, and while they fight Itsuki will sneak in at the last second and win. But Miku and Nino are best girls.
Notice how her bubbles are always hiding her (Michika? Ichiku?) earring, Fuutarou was on that side of her face, and last page you can see part of her ear.
This (dirty) move might not end well for Ichika.
All the support of her sisters pushed her too far.
Here you can see how desperate she is.
At the same time she still wants to maintain her good image as the older sister.
Wow, what a snake. How does she think that this will work? Once Fuutaro talks about it with Miku, it's going to become obvious to him that someone was impersonating her.