my perzine is multi-topical because its basically everything that is going on in my life and in my head on the page, as well as being pop culture related, i think variety is the spice of life. if a zine has stuff i'm interested in, i'll read it.
single themed issues are just as good, and serve a distinct niche. i did a comp zine about a specific issue and it was a good format that worked.
i prefer both especially with interesting content.