The thing about battle rap is it's both a sport and an art form. As a sport, you can judge it based on objective and quantifiable measures, such as use of metaphors, wordplays (and making sure the wordplay makes sense in both meanings or more if it's a triple extendre, etc) and also execution. In the case of text verses, you can even go as far as measuring syllable count in order to get a sense of flow and cadence. Judging it from this perspective is where the voter's own skillset can come into play. As a writer yourself, you can distinguish between which of the two verses would have been more difficult for you to come up with, for example, or which metaphors or wordplay would have taken more imagination to come up with, thereby judging which is more "creative."
But to the degree that battle rap is also an artform, you can also judge it from a purely subjective perspective. You don't have to understand wordplays and metaphors or be a skillful writer yourself to know which verse entertains you more. It also comes down to value as @
Malishus said. Someone can understand flow, metaphors and all the technical aspect of writing but not value it as much as how funny a verse was (jokes over bars) or how much they value personals, etc. And they have every right to their own preference. At the end of the day, what makes a good vs a bad voter isn't how skillful the voter is themselves or whether they made the "right pick". It's how well they explain their choice.