Although Mai usually wears an expressionless face, she doesn't hesitate to show her emotions when the situation really calls for it.
Mai reacting to Yuuko's excuse
Mai and Yuuko's surprised expressions (from Chapter 106).
Mai's reaction as a middle school student when Yuuko asks her an awkward question (from Chapter 134).
Mai's shocked expression (from Chapter 167).
From Keiichi Arawi's Twitter
Mio's imagination in which Mai shows emotions much more frequently, fails her exams and gets laughed at by Yuuko - the complete opposite of her actual self (taken from Nichijou - Chapter 92)
The original Japanese version of the previous picture.
Mai as she hugs the Snaq together with Yuuko and Hakase (taken from Chapter 130).
Yoshino pranks Mai after the latter tries to avoid her - note Mai shouting out of reflex (taken from Chapter 143).
Mai successfully dodges everything that is blown at her as she is reading a book down the street (taken from Chapter 157).
Mai's father(?) telling her that Yuuko and Mio have come to visit her (taken from Chapter 163).
How Mai sees Mio, Nano, and Yuuko while she has fever (taken from Chapter 163).
Mai pretending to like Yukko just to embarrass Yukko.
Mai messing with Yukko during a test
Mai trolls Yukko and the Principal with a bald cap.
Mai's first appearance in the manga.
Mai giggles while stuck in an elevator with Yukko and Mio
Mai carving, taken from Nichijou, Chapter 169
Confused Mai, taken from Nichijou, Chapter 169
Mai's troll "I'm going home" notes.
Adult Mai as she appears in chapter 185