I am not quite familiar with what SMI stands for, but if its a diagnoses you have and it interferes with your life I would not doubt it's real. What do those people know anyways? I would try not to worry about it too much since those people probably don't know what they are talking about....they could even just be trying to get a rise out of you, hiding behind a screen makes that easier for people but kind of makes them look bad since they probably wouldn't say that kind of crap IRL to someone.

Is it a support site sort of like this, or just a random internet forum...unfortunately random internet forums that aren't specifically for support will have a lot of that sort of garbage and ignorance than ones that are more geared towards people struggling with mental illness to discuss it.