Everybody, chill over mom's reaction. Look, put yourself in her place: your son is an absolute loner. Never brings friends home, let alone girls. Spends his time at home on the computer for hours. His grades are middling, so he's not an academic superstar. You know from the guidance counselor and his homeroom teacher that he's an introverted loner (remember when the teacher was inquiring if Nanami was bullying him?). Other than being on the computer, he works out (either at home or in the gym), but he doesn't have even gym rat friends.
Then you see him in the mall planting a kiss on an absolute smokeshow. One he's never mentioned. OK, if it was an average-looking girl, that would be enough of a surprise, but he's kissing a girl you'd never picture him with. (Neither could her friends, remember?) Nanami is one of the most attractive girls in their high school, after all.
Add in the whole strict Asian Tiger Mom and Japan's culture, and it makes sense. After all, if our moms saw us kissing an Emily Ratajkowski lookalike in the mall, they'd be in disbelief. There is the whole rental girlfriend thing that's a real phenomenon there. It makes sense that mom was really thrown by seeing her nice quiet son being physically affectionate in public with an absolute beauty.