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))) (. .)))
"She's a real Johnny-comelately" sounds perfectly fine.
( `` ( She ( ( 's ( ( ( a ( real Johnny-comelately ) ) '' ) ( sounds ( perfectly fine ) ) ) ) . ) ) )
(ROOT (S (`` ``) (NP (PRP She)) (VP (VBZ 's) (NP (NP (DT a) (JJ real) (NN Johnny-comelately)) ('' '') (SBAR (S (VP (VBZ sounds) (ADJP (RB perfectly) (JJ fine))))))) (. .)))