"So often I see people on all forms of social media making fun of people for having "anxiety" and "depression" when they believe they are just doing it for attention."

That is not making fun of them.

"I am not denying that some people use it for attention..."

Exactly. When they use self-diagnosed mental health issues for attention while there are people really suffering from these, they deserve to be called out.

"... but by actively demeaning anyone that claims to have mental health issues you are are increasing the stigma surrounding them."

Then they shouldn't be self-diagnosing themselves and posting about it on social media. They are the ones that painted the image of depressed people being attention seekers.

"My main issue is that for every person that's dramatising their life for attention, there is someone else that has real problems that they have never addressed."

Because people with real problems don't post about on Facebook. Which brings up your point about how you, yourself, never posted on social media or took sick days when you "felt a bit sad."