It would've helped if she'd been able to act.
What, exactly, do you find that's so inadequate about Miss Allan's performance in this? The character she's playing is a beautiful and religiously devout woman. How is she not able to execute the the part to your liking? If they had wanted scenery-chewing Greta Garbo, Katharine Hepburn, Joan Crawford, Norma Shearer or Myrna Loy as Lucie, they would have used any of these MGM contract actresses.
Okay folks, show's over, nothing to see here!
As lovely as Ms. Allan was, she was rather one-dimensional as an actress. Granted, this role didn't call for her to be anything more than sweet and kind, but she played similar roles in a similar fashion over a long career. She was not horrible, not God-awful, not an embarrassment, none of those things. She didn't need to chew the scenery to be effective, but given that she has two men vying for her hand, her character might be expected to have more depth.
But the depth of Miss Allan's character wasn't among the main priorities of the director, the screenwriter or even Charles Dickens himself. She (Lucie) is simply a straightforward character serving as the catalyst for much of the action in the narrative and for the motivation behind other characters like Carton, Darnay and Miss Pross.
This film didn't NEED a screen diva as Lucie, only an attractive woman who could competently perform her scenes; and Elizabeth Allan delivered the goods in those respects.