Why isn't "csíkos" plural in this instance?
You don't need to adjust the adjectives to the plural form when they are in front of the noun
'the striped zebras'- 'a csíkos zebrák'
'the zebras are striped'- 'A zebrák csíkosak'
Earlier we learned that Hungarians use the singular for general statements, right?
So shouldn't "A zebra csíkos ló" be correct?
I didn't report it, 'cause I'm not entirely sure.
Using the singular for general statements is generally true. But it is not an exclusive rule by any means. The singular version is also good. Here I would maybe add an "egy", to avoid saying "zebracsíkos" - "zebra-striped":
"A zebra egy csíkos ló."