The answer may be to use manual (re-) formatting: try selecting the text in a double spaced Comment (click in the Comment, Ctrl A to select) then try /Format /Paragraph / Indents and Spacing tab and select "Single" spacing.
If that works for you, the answer may be to hand edit each Comment. Globally changing all Comments would necessitate low level manipulation of the content.xml component of the file, perhaps using a macro editor; such procedure is fraught with danger, and I certainly would not attempt it if it were necessary in my case.
Each modified Comment is assigned its own unique style internally (i.e., not visible from ODT Writer when editing) and such individual internal styles would need to be identified, removed where applied, and replaced by a single style applying the desired characteristics; perhaps also it would be necessary to tidy up by removal of the now unused style definitions (I distinguish between the style definitions and the application of that style in the internal file). I have looked into this for reasons of my own; every time I decide to leave matters alone and carry on as before.
.
| Edit: Each modified Comment, even if of the same characteristics as other Comments, is assigned a unique internal style definition and application, so there is a proliferation of internal styles, each used only once. |
.
The settings from /Format /Paragraph seem be applied in the Comment, although predefined Styles do not. The definitions for Comments in the OpenDocument standards seem to permit change of typeface, Bold, Italic, Underscore, and URLs. They do not appear to permit the application of Styles.
As an aside: in a later revision of the OpenDocument standard it would be nice in my view if Comments could have simple styles applied, also some of the Fields of OpenOffice. The use of Fields would allow a Comment to point to a relevant BookMark or other Field, or perhaps have an automatic counter field.