Laughing at a janitor for their work or difficulty is disrespectful. Being employed is something to be proud of.
I think some people, not just Christians, don't get their own religion. Important philosophies don't make a connection. Also we have to battle our earthly urges to dominate other humans. You made a good point that Christians should be transparent and not have fake smiles. Basking in the holiness of God is what should make us happy, not demeaning people with blue collar jobs. I think it would be wise to pray for these people at work that are rude or dismissive.
Don't let people treat you like dirt.