What is a term for someone who is bad at communicating with other people?
Solution 1:
Does 'inarticulate' work in your context?
Solution 2:
Not well-spoken brings the point across without being too offensive. You can euphemize even further and use not too eloquent. Conversely, on the other side of the spectrum, there's not articulate.
If you need a noun, that of course would be not an orator.