Are some subjects unsuitable for humour?

I would say it might depend on the type of humour and on what effect it's aiming for. If it's just about poking fun at people who are having a hard time and are suffering, then I would say it's wrong, but if it's meant to help people overcome their suffering and provide them some comfort by pointing out the absurdity of their situation it can be perfectly acceptable.

Here is an example: It would be wrong to make fun of someone who has just been diagnosed with cancer. That would be very mean and cruel. But joking about cancer in general is not necessarily a mean thing. If you think about it, cancer is actually pretty absurd. Your body suddenly starts working against you, things start happening to your cells you have no control over. You're in pain and start asking yourself: why is this happening to me, what have I done to deserve this? Without of course ever receiving a satisfying answer. I think black humour can be helpful in a case like this.

in my eyes no. anything goes. rape, race, disabilities and sexuality included, and it can work, if u do it well. but thats hard.

BUT, if u do do it, it doesnt mean people cannot get offended or they need to find it funny. and u have to be ok with the results.

my comedy teacher asked us to be light on two things: rape and the f word for gays. (none of us would even consider the n word). And he always said that the best comedy is tragedy plus time, and the time needed before people find something otherwise tragic funny is different in each instance

It saddens me to see how many limits on humor people pose these days. its not needed. if its really not funny/working it will not work for the comedian and thats it.

oh and on the day robin williams died we were asked by the club manages to take out any references/jokes on suicide. but i thought that was appropriate and again neither of us on stage that day or even week would even want to include it that i know of.

No.

Almost always when the joke is about you and you are the victim.

I have, quite a few. Its usually those that are not "punching down" making fun of a potential victim (she is too ugly to be raped, etc) or gas-lighting someone/the problem, but those that make fun of the rape culture or of the rapists.  I agree its rare, and it has to be done well, with cleverness and also come from the right person (not necessarily only women though) but it can be done, in my opinion.