Mieruko-Chan has taken a more comedic approach to horror with a fairly great amount of success. Miko's reactions to seeing all of the different spirits -- and how she deals with them -- has led to some of the funniest scenarios in the series.
Her mentality of "Don't let them know I can see" is rather common for someone in her predicament who isn't equipped to handle some of the more dangerous spirits, or someone who's just trying to live a normal life. This mentality isn't just limited to anime, though. It actually comes from a real concept in many ghost stories.
Miko tries not to let the ghosts know she can see them mostly because she's scared of them, which is completely fair. The way they appear to her would frighten anyone who's only just begun seeing them. However, there is also an idea in many cultures that it is better to ignore the spirits -- if they know one can see them, they can attach themselves and feed on one's energy.
While Miko's friend Hana can't see them, Mieruko-chan viewers can see them feeding on her aura. That said, they don't actually physically harm her. If Hana were to acknowledge the spirits, that would allow them to do so. By giving them attention, it feeds them and makes them grow stronger. That's why ghosts at the center of rumors tend to be more powerful than a spirit nobody knows about. By completely ignoring the supernatural beings she sees, Miko is protecting herself and others by not feeding into it.
This idea is common throughout Asia, as connecting with the spirit world is a more common practice in these cultures. There is a similar concept in other areas that knowing the name of a spirit will grant dominion over it. It's similar to the exorcism scene from The Exorcist or the confrontation from The Conjuring 2. By acknowledging the spirit's name, the person can either condemn it or give it power.
When one person can see what others can't, it opens them up to more danger from the spirits -- especially if they use their powers to try and help them move on. While not every spirit is malicious, some will take advantage of kindhearted people. Zen's mother was feeding on him, just as she did in life. This occurred because they already had a parasitic bond, so she was able to do more harm to him after death. When Miko acknowledged her and told her to set Zen free, she went on a rampage. Had the Shrine Spirits not protected Miko, she most likely would have been seriously injured at the least.
While the rules for how spirits work in Mieruko-chan aren't completely clear, not every ghost is evil and wanting to cause harm. Some watch over their families and want to help the living. However, they too fall victim to the more violent spirits that feed not only on the emotions of humans, but the essences of other spirits as well. There's little that can be done for them because if someone acknowledges what's going on, they also become a target and could make the spirit even stronger. This puts Miko in a precarious position as someone who has second sight. She has to be more careful than anyone else because her powers are so strong. For now, ignorance is often bliss.