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))) (. .)))
I'd rather you use a different way to phrase, "She's a real Johnny-comelately."
( ( ( ( ( ( I ( 'd ( ( rather you ) ( ( use ( a ( different way ) ) ) ( to phrase ) ) ) ) ) , ) `` ) ( She ( 's ( a ( real Johnny-comelately ) ) ) ) ) . ) '' )
(ROOT (S (S (NP (PRP I)) (VP (MD 'd) (VP (PP (RB rather) (NP (PRP you))) (VB use) (NP (DT a) (JJ different) (NN way)) (PP (TO to) (NP (NN phrase)))))) (, ,) (`` ``) (S (NP (PRP She)) (VP (VBZ 's) (NP (DT a) (JJ real) (NN Johnny-comelately)))) (. .) ('' '')))