Thanks for asking this. Addiction is NOT mental illness - they are two very different things.
Maybe, Manziel's addiction playing out so publicly will keep some kid somewhere from becoming an addict. Maybe the sad state of Johnny Manziel self destucting on social media and being ridiculed for throwing away his career and his life to alcohol and drugs does some good, for someone, somewhere.
However, I don't feel bad for Johnny Manziel - he has chosen to be an addict. He can choose, at any time, to get into recovery. At that point, I will root for him to get clean despite all of his terrible behavior up to this point. Addiction is terrible. I hope he someday sees the error of his ways and makes steps toward making amends and changing his behavior/life. He will have a ton of support if he ever does, regardless of what his (shithead) father says to reporters.
Until Manziel chooses to try and address his addiction, he deserves no sympathy from anyone and he should be "made fun of" for his terrible, awful behavior and choices.