Asuka is starting to become the "lovestruck character who can't get the one they want" in the fandom now, when I bet if she even were to straight out confess in the manga she would be just fine hahaha
In the manga it feels like she's definitely gay for Tomoko, but not sure if Tomoko's actually gay herself / is interested.
Plus now Asuka's finding out about Yuu, Tomoko's BFF, so she might be wondering if Tomoko secretly already has a girlfriend or crush or some other reason why Tomoko's not returning her interest, even if she is gay.
All of which has really less to do with Tomoko's straightness, than the fact that Tomoko holds Asuka up on such a pedestal that it never occurs to her that Asuka might actually be flirting with her. Like anyone else, Tomoko's mind goes right to it, but Asuka is too obviously wonderful to ever be into her / be into girls.