Technically Maki, Makoto, Maya, Miku, and Molly, are all MMs, so it could be from any of them, yet it sounding like Maya to Maya is interesting.
The Smiley Face seems more of a generic thing, however both the Wire god and Himawari/Shapeshifter tend to use or talk about smiles, Pareidolia does as well (basically uses whatever it seems), but seems to prefer a neutral face, while HOPE prefers an upside down smiley face, and I guess Nao-chan uses a diagonal mouth face(?):
Pareidolia does seem to use more messed up smiles and things so it could be an indication of it. Haruka's name also involves Haru which translates to Spring (and she's also a predator), so I do wonder if she's involved with Pareidolia in some way.
What is with all the smiles? Well, according to Himawari/Shapeshifter:
Not entirely sure why, beyond how smiling can be an indication of happiness even if one isn't happy. Maya and Ami both seemed to want to know if Sensei was "happy here" early on:
Which seems to be somewhat important considering it's also a thing from 'Nothing Is Beautiful':