There is nothing ungrammatical or wrong about saying  She's a real Johnny-comelately,  but it grates.
( ( ( ( ( There ( is ( ( ( nothing ungrammatical ) or ) ( wrong ( about ( saying ( She ( 's ( a ( real Johnny-comelately ) ) ) ) ) ) ) ) ) ) , ) but ) ( it grates ) ) . )
(ROOT (S (S (NP (EX There)) (VP (VBZ is) (ADJP (ADJP (NN nothing) (JJ ungrammatical)) (CC or) (ADJP (JJ wrong) (PP (IN about) (S (VP (VBG saying) (SBAR (S (NP (PRP She)) (VP (VBZ 's) (NP (DT a) (JJ real) (NN Johnny-comelately)))))))))))) (, ,) (CC but) (S (NP (PRP it)) (VP (VBZ grates))) (. .)))
Technically there's grammatically incorrect in the sentence, "She's a real Johnny-comelately."
( Technically ( ( ( there ( 's ( grammatically ( incorrect ( in ( the sentence ) ) ) ) ) ) , ) ( ( `` She ) ( ( ( 's ( a ( real Johnny-comelately ) ) ) . ) '' ) ) ) )
(ROOT (S (NP (NNP Technically)) (PRN (S (NP (EX there)) (VP (VBZ 's) (ADJP (RB grammatically) (JJ incorrect) (PP (IN in) (NP (DT the) (NN sentence)))))) (, ,)) (NP (`` ``) (PRP She)) (VP (VBZ 's) (NP (DT a) (JJ real) (NN Johnny-comelately))) (. .) ('' '')))