Yeah, I think it is common in some regards.
The current prevailing theory on how synesthesia happens in the human brain says that everyone is born with these connections but in early development the synaptic connections are sheared or closed off. Like how your brain will close off bad memories or certain emotions for trauma victims. It is believed to be a similar mechanism. The connection is still there but it is shut down and inaccessible. When people take hallucinogens and see colors or geometric shapes in their vision that is the drug reopening those closed synaptic connections temporarily.
The degree in which synaptic shearing happens or which synaptic connections get sheared are different for every person. So there could be, like, a bell curve, where some people have more connections throughout their life than others but the majority tend to group towards the middle of the curve. For mirror touch, and several other types of synesthesia, it tends to be a bit of a spectrum of those who don’t experience it to those who experience it acutely. Even then there are those who may experience it internally, like intellectually know there is a sensation and those who have some sort of physical reaction. Synesthesia presents itself in wildly different ways among those with the condition. There are many triggers and many different kinds of responses. If you have one type you are 50% likely to have 2 or 3 different types of synesthesia.
There are two main tranches that people with synesthesia fall into in regards to the affects of synesthesia; people either think everyone has the same thing and so they don’t talk about it or, once they do talk about it, discover not everyone experiences the same thing they do, or not to the degree in which they do. If the synesthetic response is strong, hard to ignore and not something people are used to hearing about then the synesthete (a person with synesthesia) tends to not talk about their condition due to fear of being ostracized or sent to a psychiatrist and etc.
I have Emotion>Color Synesthesia, along with several other types (including mirror touch). So my emotions present as colors projected into my vision. When talking about it as a kid I was sent to a psychiatrist as my parents thought I was making it up or crazy. I was put on anti-depressants which, surprise surprise, dulled my emotions and made the emotional>color synesthesia less pronounced but still there. Emotion wasn’t considered as a “sense” in the traditional meaning so it took a long time for me to figure out that it was synesthesia and how mine worked. Only in the later years of the 2000s did synesthesia start to come back into scientific research circles and with the internet it made it possible for edge cases, like myself, to start making contact with each other.
If you want to learn more about it then the book from 2009 called “Wednesday is Indigo Blue” is a great starting point and kinda spearheaded synesthesia coming back into public discourses. It is not a long book, gives a good overview of the condition and its many variations but is also approachable and not too heady.