There is no "normal" person. Everyone is f*cked up in some way. It's a spectrum and depends how bad the disorder is before it starts adversely affecting your life. People can live with all sorts of s*it like bad anxiety, addictions, BPD, NPD, etc. Once the disorder starts adversely affecting a person's quality of life -- or worse --affecting other people's lives, then it becomes a problem.
Mental illness is just as real as a thyroid, pituitary, or liver condition. Only with mental illness, it's largely a problem with a chemical imbalance or mis-programming/wiring of the brain due to injury, heredity, or trauma--usually at a young age. The brain is quite plastic so behaviors can be "unlearned" and the person reprogrammed, but it takes time and a ton of effort and the person has to want to change. Generally, for severe mental illness, the Rx is counseling + meds to compensate for the lack of the proper neurotransmitter.