Here's a little snippet from the documentation of lemmy.ml and lemmy.world. I understand it as meaning that the downvote is actually there to express a negative opinion about something you don't like.

Lemmy uses a voting system to sort post listings. On the left side of each post there are up and down arrows, which let you upvote or downvote it. You can upvote posts that you like so that more users will see them. Or downvote posts so that they are less likely to be seen. Each post receives a score which is the number of upvotes minus number of downvotes.