This is the kind of development I would welcome in the original series. I'm not above admitting that I'm frankly charmed by Manako in this chapter, and with recent developments in the original webcomic, it's something I could maybe see happening much later on.
But.
At this point in the story, it mischaracterizes Saitama. Again. In multiple ways. The filler Saitama might as well be a different person from canon Saitama. It's so annoying.