There is merit to trigger warnings, provided they are used correctly. As an example one of my professors told us a story about a student he once had. This girl, unbeknownst to him, was a former heroin addict, and during one lecture he happened to start speaking about heroine and its effects, as well as showing images. A few days later he found out that this small thing had triggered a relapse for this girl, and that she had to be rushed to the hospital for OD'ing. Since then he's always used trigger warnings when it comes to sensitive material like that, in hopes to avoid something like that happening again. And I think when it comes to something like that, he's correct. When you're speaking to a large group of people, whether in a lecture or online, you're going to bump into people that have some kind of extreme adverse reaction to something like that, whether it be drug abuse, sexual abuse, etc... and it's best to mindful about that sort of thing. Because people who actually have cause for trigger warnings don't just get 'uncomfortable' with the source material, but they can have pretty damaging effects on their physical and mental state.
And that's where the problem arises.
'Trigger warnings' don't even really mean trigger warnings anymore. It's a term that's been morphed from, 'I will have a genuine physical and/or psychological reaction to X and as such need to be warned beforehand' to, 'I don't like X/it makes me slightly uncomfortable, I don't want to look at it.' That's not a good thing, and it detracts from people who do need them, turning their warranted 'please let me know beforehand' into the butt of a joke. I've seen trigger warnings for just about everything online, two of my favourites have been 'TW: Cupcakes' and 'TW: DBZ', though the one with the pomegranate that someone demanded be labelled with 'TW: Heart' is also a pretty good classic. Of course I don't know what these people have been through, but I doubt cupcakes or buff anime men actually cause you to dissociate and if they do, you require medical attention. The people who 'require' such trigger warnings should be looked at with scorn, because not only are they making themselves look like idiots, but they're also tarnishing the idea of trigger warnings.
All that said, I'm not going to be an asshole on purpose. If someone asks me for one, I'm not going to kick up a fuss. But that doesn't mean that I'm not going to take a screenshot of the whole thing and post about how absurd it is later. (That is, if the request for the trigger warning is in fact crazy.)