I asked this from my dietician and naturopath, both said that small amounts should be ignored and after a few conversations and over the moths I developed my own rule, any more than 10% of mucus and persisting for a couple of days I would start to pay attention. Of course if it were a food reaction, it would be coupled with unsettledness.
It is possible to have a say a 50% mucus nappy and a very happy baby - in that case ignore. In our case though, mucus was usually accompanied by poor sleep and irritability.