Dickens seems to be guilty of a bit of prejudice:
"The
third party, which had ascended from the valley on the Italian side
of the Pass, and had arrived first, were four in number: a
plethoric, hungry, and silent German tutor in spectacles, on a tour
with three young men, his pupils, all plethoric, hungry, and
silent, and all in spectacles."

A "plethora" can mean an abundance, an excess of something.
So a plethoric person is stout, looks overweight, and possibly hypertensive. I doubt that spectacles are more common in Germany than in England.

Webster's 1913 Dictionary

Definition: \Ple*thor"ic\, a. [Gr. ?; cf. F. pl['e]thorique.]
Haeving a full habit of body; characterized by plethora or
excess of blood; as, a plethoric constitution; -- used also
metaphorically. ``Plethoric phrases.'' --Sydney Smith.
``Plethoric fullness of thought.'' --De Quincey.

So, they're a party of fat hungry bookworms who sit on their arses all day, eating junk food, except when touring when all they think about is food and identifying things they've read about so they can make plethoric observations.

I don't think Dickens was prejudiced at all. He just knew a type when he thought of one!