Meaning she had a wide or crooked mouth. Holden's talking about Jane Gallagher, whose muckle mouth he likes very much. At least she wasn't named for this feature, as Robert Browning's Muckle-Mouth Meg was:
“Life’s sweet; shall I say ye wed Muckle-mouth Meg?”/ “Not I,” quoth the stout heart: “too eerie/ The mouth that can swallow a bubblyjock’s egg:/ Shall I let it munch mine? Never, dearie!