I don't see a way to edit this style property in the iOS version of Pages. (I'm using 4.0, March 2018). Am I missing it, or do I need to open the document on my macOS system to set this property?
Unfortunately, as of the current version (Pages iOS 4.0) it is not possible.
Line height is supported but not spacing between paragraphs.
However, in addition to the macOS version, iCloud Pages can do it too.
Adjust the paragraph spacing:
You can control how much space appears before or after one or more paragraphs.
Click to place the insertion point in the paragraph you want to adjust, or drag to select multiple paragraphs.
In the Format sidebar on the right, below Spacing, click the up or down arrows, or enter a point value in the adjacent fields.
You can adjust the margins for specific paragraphs to indent them from the document margins, and change the indentation of the first line of each paragraph.
You can also increase or decrease the spacing between lines in a paragraph.
From the Format sidebar, click the arrowhead next to Spacing. That will reveal the Before Paragraph and After Paragraph options. Then click the up or down arrows, or enter a point value in the adjacent boxes.