As OP mentioned, sometimes a slur might be needed for a comic that's handled maturely in order to portray an act of prejudice between characters realistically. Sometimes you just need to show how much of a mean character someone is and having them use a slur can make sense. If your work is set hundreds of years in the past, yes, slurs will be used because a long time ago these words were not considered slurs and were commonly used. People with certain disabilities used to actually be referred to as "idiots" in the medical field with no negative intention, whilst now it's not such a neutral thing to call somebody. Context is important and in the right context not using a slur might be unnatural...
Alongside that, using it as reclaimed language is pretty positive and harmless. A character facing a slur that's used against them head on and being proud in the face of it doesn't harm anybody.
However, I don't really think it's at all okay to just throw them around. They should never be used light-heartedly or by characters we're supposed to really like. They shouldn't just be used with reckless abandon.
Using racial slurs and not portraying it as distasteful? Throwing around transphobic slurs like the T word or the dreaded "tr*p" whilst showing no concern for how hurtful or damaging the words are within context? That's not cool and can be uncomfortable to read. Not being able to tell if a work is expressing hostility towards you can be extremely off-putting and make you not want to read anymore.
It's also obviously important to treat it as profanity since that's what it is. I can't stand seeing people use slurs as if they're not inappropriate or rude words... They are. If a comic is meant to be for teenagers or kids it's just not appropriate. If it's a light-hearted comic it's just not appropriate.
There are a few instances where characters of mine use slurs or hurtful words - contextually bad people. These are people who hurt others and are generally disliked. If a character uses a slur or says something hurtful it's because they're not a thoughtful or kind person, and the same applies to a fictional character.