A while ago I used to have my own tiling business and I took it upon my self to do the plastering also. But when I had finished the plastering the customer commented thus 'I have seen better jobs on the side of a toilet.'
It means that plastering the outside of public conveniences is not considered a priority job by Councils, so they sometimes employ cheaper - and substandard - workmen to do that job.
Your customer is advising you that they have formed a low opinion of your plastering skills.
You can't really 'take it upon yourself' to do plastering, it's a specialist trade and takes years of practice - a perfect example of something being about as difficult as it appears to be easy.