To me true lyricist should be like a poet. His words should be intelligent but clear, he should rap about existence problems, go deep with his raps and compone them with good wordplay.
I think it's impossible to define lyricist. For one it will be a intelligent rapper going deep, having good wordplay (my opinion), for others it may be just rapper talking about problems they're concerned with or a rapper having nice wordplay but not necessirly going intelectual or deep.