The mini's growth is determined by the amount of care mistakes, so yes, how you take care of your Tamagotchi determines what character it becomes. The mini is a little difficult to understand if you haven't had a Tamagotchi before but thankfully, it can be easy to care for.
Basically, just like a regular Tamagotchi, the Tamagotchi mini has a meter for hunger and happiness. These meters have four bars each and each bar is filled with one serving of food. Feeding the tamagotchi bowls of rice will fill the hunger meter and feeding it candies will fill the happiness meter. The problem is, these meters are hidden so the only way to tell if that meter is full is to feed it till starts shaking its head. If the meter empties, a care mistake is earned and the amount of care mistakes determine growth. If the hungry meter empties, the tamagotchi will look spaced out with a swirl in the air. If the happiness meter empties, the tamagotchi will turn away and a throbbing symbol will be in the air. Another way to get care mistakes is to not clean up after the tamagotchi. Enough messes on the screen will cause it to get sick and unless it is healed, it will die. Additionally, letting it sleep in messes also counts as a care mistake.
Now that the mechanics have been explained, here is a basic care schedule:
1. When the tamagotchi first hatches, feed it 4 rice bowls and 4 snacks.
2. An hour later, feed it till it shakes its head. (It loses a bar approximately every 45 minutes. Fully fed, it can definitely go without food for an hour.)
3. Clean up any messes it makes.
*To check if a meter has fully emptied, press C (button on the far right). If its meters have not emptied, nothing will happen.
*To put the Tamagotchi in limbo, press B (middle button) to bring up the clock screen and then A (far left button) and C to put it in set mode. The tamagotchi will not do anything till the clock is taken out of set mode (useful for times it cannot be taken care of).
Do not feel like you have to be super, super attentive to avoid Nyorotchi. Even with 10 care mistakes, I still got the best character. The key is to check it at least every hour and feed it till its full. The more full the meter, the more time you can leave it alone. Also, as minis are small and all its needs are taken care of with a few button presses, it can be easily cared for without disrupting your life. One other thing is that there are secret characters which evolve from adult characters. If Nyorotchi is treated well, it will evolve into one as will Mametchi with the same care.