No, it becomes a different series entirely if they just start fucking like that. Even them going on dates is treated light hearted or like a joke so far, with them accidentally asking the other out, or it being a misunderstanding. The TT aspect of it is that even if it was a misunderstanding, the other one still agreed. If it was mutual and not a misunderstanding at all it wouldn't be a comedy series anymore. In the end everything in this story is filtered as a comedy. Even with Suika cutting Momo up, you still had Shamiko break the tension by doing something stupid twice in that moment.
Knowing this concept and how the series works, you have to have the relationship advance in ways that actually fits the story
They kiss by mistake because Mikan bumps into them and they fall into each other
Due to a misunderstanding Momo thinks that Shamiko is asking to have sex, and Momo gets ready for it, but ultimately realizes at the last minute that Shamiko was just asking Momo what she wanted for dinner that day
Shamiko day dreams about having a kid one day, and in her imagination the kid strangely has pink hair.
That's how you have them stay in character while still advancing the relationship in subtle ways.