No good at spelling or grammar? My friend, that's the the least of the things you are no good at.
You're no good at disguising the fact that were very badly brought up.
You're no good at disguising the fact that you're a human failure.
You're no good at disguising that you're a racist.
You're no good at disguising the fact that your parents must have been inadequate as well.
You're no good at being good.
You're no good at avoiding leaving yourself open to the charge of mental deficiency.
Somethings you are good at.
You're good at displaying where your level of humour lies.
You're good at displaying your personal inadequacies.
You're good at leaving yourself open to the charge of being a lacklustre cretin.
You're good at showing yourself to be white trash.
You're good at showing that you're proud of your stupidity.
Yes, it's amazing what information one can glean from a few sentences worth of blather. Obviously, in the case of some of the above observances, you don't want to disguise your appalling characteristics.