Why would eternity show negative comments?

I may be wrong, but I believe that may reflect deleted comments

Most likely this is reading the values from the aggregates table which is provided by the server api - that table can get in a broken state for a variety of reasons, mostly just missing validation.

My guess is someone tried deleting their post, the request hung so they pressed the button repeatedly, one request removed the post and the rest kept decrementing the aggregates table.

Lemmy be dumb sometimes

Negative comments on Lemmy usually reflect stuff that have been deleted.