anti-capitalism gets classified as a mental sickness

It doesn’t. People are making shit up. From Wikipedia:

The fourth revision of the Diagnostic and Statistical Manual (DSM-IV-TR) (now replaced by DSM-5) states that a person must exhibit four out of the eight signs and symptoms to meet the diagnostic threshold for ODD. These symptoms include:

• Often loses temper

• Is often touchy or easily annoyed

• Is often angry and resentful

• Often argues with authority figures or, for children and adolescents, with adults

• Often actively defies or refuses to comply with requests from authority figures or with rules

• Often deliberately annoys others

• Often blames others for their mistakes or misbehavior

• Has been spiteful or vindictive at least twice within the past six months

These behaviors are mostly directed towards an authority figure such as a teacher or a parent. Although these behaviors can be typical among siblings, they must be observed with individuals other than siblings for an ODD diagnosis. Children with ODD can be verbally aggressive. However, they do not display physical aggressiveness, a behavior observed in conduct disorder. Furthermore, they must be perpetuated for longer than six months and must be considered beyond a normal child’s age, gender and culture to fit the diagnosis. For children under five years of age, they must occur on most days over a period of six months. For children over five years of age, they must occur at least once a week for at least six months. If symptoms are confined to only one setting, most commonly home, it is considered mild in severity. If it is observed in two settings, it is characterized as moderate, and if the symptoms are observed in three or more settings, it is considered severe.

Is this how you express your anti-capitalist and anti-authoritarian stances?

I could count under at least 4 of these and I consider myself an anti-authoritarian but otherwise normal, well-rounded person.

I do get easily annoyed by some people. I get angry and resentful when I think about the current state of politics. And I do get spiteful when authority figures ask me to do something ridiculous (malicious compliance, basically), which I guess is a 2-for-1 there.