You can configure the caption component of the figure component to your liking using #show. This affects of course only the captions!
#figure( // Typst default
image("cat.png"),
caption: [This is a cat!]
)
#show figure.caption: set text(style: "italic")
#figure( // Typst default with italic caption
image("cat.png"),
caption: [This is a cat!]
)