Just had a random thought. It felt really weird that Micchan would just act all salty towards Marina like that for no apparent reason (other than being tsundere, I guess), but maybe she got upset because everyone else's moms showed up, and they didn't have anyone to speak of.
You know, we may just be overthinking this, but this light slice of life series gets really depressing the moment you stop to analyze the subtle hints Norio-sensei drops every so often.