This manga is best used as a source of laughter.
Apparently over in Japan girls who read manga have a height obsession as intense as Japanese otaku boys' big breast obsession. Thus it has become a convention in shoujo manga to have the boy be at least a head taller than the girl, and often skinny enough to blow over in a wind to boot.
In this case, though the boy is several heads taller than the girl. They look silly, almost grotesque, together, and I suspect tjat managing a kiss standing up would be a major contortion struggle for them. Conventions in popular fiction are hard to deviate from, and this mangaka didn't bother to here, when she set out to write a story about an extra-short, shy girl. That's fine, I guess, for the girl readers--although I bet even some of them are put off by the sheer unliklihood of such a coupling.
As for boy and men readers, since very few of us are several heads taller than the girls around us (especially in Japan!), the vast height difference between the boy and girl in this manga makes it too hard to identify with the main guy in this story, and too easy to laugh at the grotesqueness of the whole proceedings.
True, the manga is not aimed at us. But if you're going to set out to write about an extra-short girl and her struggles, it would seem reasonable to shorten her bf a bit. I would think if these two were somewhere in the same height ballpark the relationship would come across as more equal and companionable.