Harold and Alberta Scrubb are Eustace's fad-loving parents. Here's what the narrator has to say about them:
They were very up-to-date and advanced people. They were vegetarians, non-smokers and teetotalers and wore a special kind of underclothes. In their house there was very little furniture and very few clothes on the beds and the windows were always open. (1.1)
Do you hear the sarcasm there? What bothers our narrator about the Scrubbs is not exactly the choices they make, but the fact that they jump on bandwagons without really considering the effects of their decisions. The narrator of The Voyage of the Dawn Treader and C.S. Lewis himself both think that time-honored values and activities are worth preserving and shouldn't be dumped just because someone invented a new undershirt. Throughout the book the narrator will imply that Eustace's unpleasantness is the fault of his parents, who have a lot of funky ideas but not a lot of deep values.
Just to clarify, Shmoop is totally down with vegetarianism and leading a substance-free life. Social attitudes and scientific knowledge about health were different back in C.S. Lewis's day. Think of him as feeling like his world is being threatened by new fads; that's why he's making fun of them.