Well to answer this question I had to do alot of thinking. I never even considered that mediocrity could be caused by negative emotions, I just felt that some people aren't good at certain thing, but upon further thought, I came up with these three and I'll do my best to explain why I feel so
1. Sadness: Sadness is often characterized by, self critical thoughts, pessimistic thought, and thoughts of loss and failure. When people attempt to do certain things, maybe learn to play a piano, sing, or dance, alot of factors determine how good you can be at that particular thing, but your mindset is what matters most. If you're pessimistic about your capabilities then you're only going to achieve as much as your mind says you can, and if you're constantly worried about failure then you will subconsciously make yourself fail or at least prevent yourself from achieving greater heights. Sad people often show high levels of mediocrity, maybe that's why they say whatever your job is, it should be something that makes you happy.
2. Anxiety: Anxiety is characterized by what if thoughts, like what if "I'm not good enough?" or "what if they don't like me?". It's thoughts like that that prevent people from becoming better at what they're doing. The mind has a limiting ability and a limit breaking ability, if you let the anxiety of possibly failing or becoming less than expected take over you then you will undoubtedly end up as a mediocre in whatever you're trying to be.
3. Shame/Embarrassment: This particular emotion only comes into play when you've actually tried to do something and failed at it and you feel that people will ridicule you for not being better than you are. The fear of embarrassment has probably hindered alot of people from taking the bold steps they need to take to go from average to great and since they can't take those steps they end up staying mediocre.
This is how I see things and I hope it answers your question.
Psychology could cause mediocrity and bad experience could lead to not make any action to new opportunities.
Maybe there are more than three but let's write three of them:
1- Being Afraid: People are afraid to seem like unsuccessful and they do not make any new action. This is due to possibility of encountering bad results. They choose to be an ordinary person and not try anything.
2-Feeling Tired: They could have many experiences and that is why that could lead them be a lazy person due to the failing before
3-Being Inpatient: Some people do not have enough energy to complete anything they have started. That causes being bored and killing excitement they have.
These could be reason to mediocrity and could lead people behave like that.