Yes! You can say that too if they are not good at what they do.
And you can refer to objects too, like "these mousepads are not good".
- Aqueles jogadores não entraram para o time, eles não são bons = Those players didn't enter the team, they are not good (players).
- Não quero estes biscoitos, eles não são bons = I don't want these cookies, they are not good/tasty
I know it's been a while since you posted this, but I just thought of one other example besides "no good" that's commonly used in conversational English: no help (meaning unhelpful).
As in, "Well, that was no help at all." Or "The kids hate to clean up, so they're no help around the house."
The correct translation for "they are not ok" is "eles são estão bem". The verb 'to be' has two usual translations in portuguese: "Ser" and "Estar". The first one, 'ser', is used when it's subject is something in a more static condition, like an essence of the being. You can say that "you are inteligent (você É inteligente)" or "They are capable (eles SÃO capazes)". Altough, the verb 'estar' is used to express a temporary or easily changeable situation. For example: "You are tired (você ESTÁ cansada)" or "I am here (eu ESTOU aqui)"
Then, if "they are not ok", it would be "eles não estão bem". If "they are not good", it's "eles não são bons". The verbs call for the right adjectives, "bom (good)" for "ser" and "bem (well/fine/ok)" for "estar".