Due to misuse and mockery of the term, a lot of people now prefer to use "Content Warning" rather than "trigger warning", particularly as certain content can be upsetting for people outside of those specifically with ptsd.
Things I'd expect from a mature rated comic would be: Swearing, nudity, sex, light kinks, death and violence, probably with some blood and injury.
Things that might need a content warning for that may not even be in a mature tagged comic:
- Sexual assault and sexual violence. (A common cause of PTSD and... sadly not all that uncommon statistically)
- Extreme depictions of descriptions of gore.
- Transphobia and homophobia. (Magical Boy is an excellent example of a comic that, while it's mostly a fun, lighthearted adventure, goes heavy and very real on depictions of dysphoria and what it's like to experience transphobia. Even as a cis person I found it emotionally intense! The content warnings are sensitively written and very necessary to help the reader feel emotionally prepared or know that it will upset them).
- Racism (some people have to put up with that shit on a daily basis. They should be allowed to avoid it in fiction if they just need some escapism)
- Drug Use (for people who are recovering from addiction or were abused by somebody in their family or friends who was under the influence of drugs it might be nice to have a warning)
- Suicide and self-harm
