The well-off people of London Town are mostly human - but most of the poor people are Muppets.
...I don't get it.
Except they aren't. There are barely any human characters we see who are rich. Fred and his wife are said to be poor. On the other side, the pigs are shown to be wealthy, the Marley brothers were certainly rich, as was Fozziwig.
See those human actors walking around in Victorian-era clothes in the crowd scenes? They're not just extras. Most of them are the puppeteers. Look closely and you can just about see the muppets some of them are walking with. Brilliant.
The Ghost of Christmas Past is the only one with a defined form. That is because the past cannot be changed and will always be the same. The Ghost of Christmas Present ages because the present is ongoing and constantly changing. The Ghost of Christmas Yet To Come has no face because the future can't be known.