Well, there's a difference between laughing at and laughing with. It doesn't necessarily mean mocking. Secondly, mocking can be, well, pretty damn hilarious sometimes. Putting a blanket statement over the subject or dividing it into a dichotomy doesn't help at all. I guess it's all about where you as an individual draw the line and whether you feel it should be encouraged or tolerated.
When I feel like it, I'm pretty lax about my issues with mental health and suicidal behaviour and what not. Sometimes I just have to joke about it and laugh about the subjects, I'm not that serious a person. More importantly, I'll go a bit crazy/crazier if I don't get some release. It's fun - people who can relate go along with it and have a laugh as well, people who don't relate or are more serious about the matter tend to feel uncomfortable or go into 'saviour mode', which tends to be a whole new source of joy for me. Just because I feel like dying nearly constantly doesn't mean I can't have some shits 'n' giggles! That's just how I am.
Of course, I have to be in the mood for all this. Sometimes I'm not, sometimes I don't feel like making a mockery out of such an issue. But that's called variation.
Is mockery dehumanising? Sometimes it is, sometimes it isn't. Like with most things, I believe on evaluating this on a case by case basis. We just have to be careful about our criticisms and try be vigilant in educating those who are clearly using the harsh struggles of others for a quick profit, whether it financial or social. Blanket statements and knee-jerk reactions don't help at all and no one learns anything.
I think the bigger question is when it comes to public figures and the like. I think one's position has to be considered when talking about what is essentially a form of social censorship. I sure as hell wouldn't tolerate a politician making light of suicide, for example. A high school principal would fall under that same umbrella as well. In general, I'd expect higher of someone whose job it is to serve others and look out for them. Not that most politicians do that, but I'll leave my cynicism out of this. The point is that a person in a chosen role of leadership needs to hold their public face higher than others.
Entertainers, however, I don't really care. Case by case basis. People can be as morally righteous or indifferent as they want to be. These entertainers have the right to free speech, but not the right of freedom from consequences. Refer to the infamous Michael Richards incident - his career was absolutely destroyed after using racial slurs and was subsequently crucified in the court of public opinion. He saw it as comedy, the public did not.
Ultimately, whether it is 'okay' comes down to what a person thinks on an individual level and what they'd expect from others. If a significant amount of people consider your actions or opinions to be cruel, then perhaps consider what is being said and whether your right to a cheap laugh is worth it.