Do you think its okay to say you have a mental illness of some sort without being diagnosed?

Obviously you can be depressed and not have depression, or be anxious and not have anxiety, as they are feelings/emotions... so that's not what I'm talking about. I'm talking about people saying that they do have chronic anxiety, bipolar disorder, chronic depression, or any other mental illness without having been diagnosed with it. My three older siblings and my mother all have diagnosed mental illnesses, and I feel like they're taken less seriously and almost made out to be not that big of a deal today since some people think they can just say they have a mental illness. Probably apples to oranges here, but you wouldn't say you had a physical illness of some sort without being diagnosed. So why are mental illnesses looked at differently? This is related to the abuse of emotional support animals to me as well, people abuse that since it can be pretty easy to get one so people pretend they medically need a support animal when in reality they don't and just want to take their dog into stores or have them in a rental when they aren't supposed to. It's making it harder for the people who actually need one, to have one.

I see people saying they have an illness on social media more than anything, they share all those links and stories titled "what it's like to love a woman with chronic anxiety" or something similar, and like with my cousin, I asked her if she had seen anyone about her social anxiety and she said she hasn't and doesn't plan to, but still says she has it.