@Syhris & @tdaniel in the Codex, Bad Skill is a term that is used for the Special Skill or Talent of Trinkets and some Pets. I am sure you have noticed that some of the Slime Pets have Bad Skill and others have Boost Element (pick one Element). I think that when there was an update and a few Slimes were added to the list, the older ones were listed with Bad Skill, and the newer ones were listed with Boost Element. the Bad Skills for the Trinkets are Silence, Slow, and Root.
when I was looking for information, I thought it was a totally different skill than what you see on the Trinkets and Slimes, but eventually I figured it out.
this is the last Post in a Thread I started that resolves the mystery of just what is the Bad Skill?
@Shield_Hero I have noticed that also, and sent in a Bug Report a long time ago. I recently did some more testing and there was still no noticeable increase in DMG or in the Resist of the Element. for the DMG, I was looking at both the DMG for Skills on the Stat Page and DMG done both in the Campaign and with a Test Dummy. +10% Element DMG for a specific Element doesn’t seem like much, but every little bit can help at higher floors, and quite a bit for new players lucky enough to get a slime with an Element they like to use.