What if the DLC is use to add, i don´t know, extra boss fights that didn´t make it in the final game (and where not intentionally leave out) because of time, and also free of charge ? Is that still a bad/good DLC ?
Extensions (free ones, at that) to a, for the most part, completed game are fine.
That's why things like DLC "After Episodes" (post story add-on content) are okay, even if they cost money.
Because regardless of if you play it or not, that doesn't hinder your experience in the actual game.
Here's another ex.
Let's say swimming wasn't included in FFXV, but a DLC update comes out for it.
1.) The update is automatic/free to download. Good DLC.
2.) The update costs $5.99. Bad DLC.
Ex. 1 is good because you have the option to acquire a gameplay element that more or less should have already been in-game.
Ex. 2 is bad because if you don't spend the extra money, you miss out on an entire element of the gameplay that should have already been in-game.