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."
Kind of. In this case, the 'n' is responsible for the toning of the 'o'. Without the 'n', the 'o' would be an open vowel, as in 'shopping'. The 'n' makes the 'o' being enounced in a nasal tone, as in 'phone'. That is the only speaking evidence of the 'n' in the word.