I don't think it is offensive, it's just a play on words. It's not mocking or trivialising a serious medical condition. Is it ignorant? Possibly, if they genuinely don't know that "OCD" has a medical meaning then I guess you could claim they were "ignorant" of that. Ignorance is not a crime though, nobody knows everything.
On the point of "OCD being the only mental health issue that is allowed to be mocked" - that is utter bullshit. As someone with depression, I see this illness being mocked, parodied or ridiculed all the time. Maybe it's not so obvious to people who don't suffer from it. But I generally don't find myself offended by it, it's usually just meant as humour.
It's a bit like when someone says "I'm going to slit my wrists." Usually they are just using it as a figure of speech to highlight a desperate or unfortunate situation. But it doesn't offend me, or remind me to think about committing suicide, because that's not the person's intention.
Back on the OCD cakes front - isn't it possible that the person behind this advert is genuinely suffering from OCD themselves, and finds humour the best way to deal with it? When I am at a low ebb I often make jokes about suicide which to someone who doesn't know me will sound in poor taste. But that's the way I deal with it, through humour. I'm not attacking anyone, insulting anyone, I'm joking about my (potential) suicide in order to make myself realise how ridiculous I'm being. The OCD cake person might be doing the same thing you know, and if humour helps them deal with their condition, what right does anyone have to tell them they are being "ignorant" or "offensive"?