If your mirror neurons don't form properly, you never develop the ability to accurately interpret the feelings or even the facial expressions of others. This can lead to intense social awkwardness as you grow up, because you can only interpret people's literal statements and not the body language that provides the context for them.
Have you ever misinterpreted an email because you projected some emotion onto it without really needing to? This happens because an email doesn't come with facial expressions to help you interpret it, which is why people started using emoticons. People whose mirror neurons never developed properly are always in that same situation, because they can't correctly interpret facial expressions or body language in any conversation. They are forced to go through life with a greatly reduced ability to understand or communicate with others. The first year of life is truly one of the most important stages for your overall happiness and well-being- so make sure to give your babies lots of love and kindness!