What makes Joesph Strorm a satirized character in The Chrysalids?
To satirize something or someone is to render them laughable through exposing the ridiculousness of their behavior. But not just any random behavior. The satirist is interested in showing how a group or an individual publicly states adherence to a code and makes a show of following the code while actually breaking the code. The tool of the satirist is wit and humor, a good jest that will produce healthy laughter in the audience.
The nature of The Chrysalids is not that of a satire. The reader doesn't think what goes on in Waknuk is at all amusing. The reader especially does not think that Joseph Strorm is at all amusing. One would be hard pressed to find anything of significance to laugh at in The Chrysalids. So, overall, The Chrysalids is not a satire and Joseph Strorm is not a satirized character. That being said, there are isolated brief incidents that author John Wyndham tells the reader about through the narratorial character of David Strorm that render Joseph Strorm ridiculous.
One such brief recollection of a past incident that does give a short satirical view of Joesph Strorm's behavior and attitude is recounted in Chapter 4. Uncle Angus Morton had just bragged about his horses being exceptionally well built, which awoke Strorm's ever vigilant attention to possible mutational deviations. Strorm had called in the town inspector who had assured Strorm that Morton's horses were selectively bred by the government and contained no mutations.
A past incident is then recalled in which Strorm had been certain that a particular cat that had no tail was a mutation. Incited to rid Waknuk of the mutated deviant cat, Strorm killed it. He was then informed that the cat was a member of a legitimate breed. This story, though a brief aside, satirically renders Joseph ridiculous making his fanaticism irrationality clear and apparent. Although the majority of towns people can't really laugh at him the way Morton does, the reader gains a wider perspective and can laugh at his extremism for at least a little while.