What makes the entry so odd is all of his self editing. Here is a transcription, pay special attention to the words and phrases he crossed out as he wrote:
While I was there Lady Byron came in. She isuncommonlypretty—not beautiful—for the prevalent expression of her countenance is that ofa simpleingenuousness. “Report speaks goldenly of her”. She is amarchionessbaroness in her own right, she’s a large fortune, is rich in intellectual endowments,an extraordinarya mathematician,(so that few of their men of science are equal to her)writesgoodpoetry, possesses all common accomplishments in an uncommon degree, and adds to all this, a sweet temper. She was dressed to go and drive and after stopping a few moments, when to her carriage. Lord Byron’s manner to her was affectionate, and when she went away he followed her to the door and shook hands with her, as if he thought it would be a months before he should see her again.
Wow, those are some serious modifications. It appears at first he was awestruck: "uncommonly" pretty; "an extraordinary" mathematician on the level of the "men of science"; a "good" poet. But then he crossed out all of those qualifiers and reduced her to a pretty mathematician who writes poetry. Well, okay, but what changed from first draft to second? Perhaps he simply couldn't abide by the fact that a woman chould be so accomplished and on the level of a man. Or perhaps the force of her personality hit him with one impression that was then tempered by time. He should have been impressed with her mathematical skill--she taught her daughter Ada, who would later be credited with creating one of the first computer programs. Regardless, it is clear that Ticknor was conflicted and had difficulty coming to terms with a woman of her standing.
To see the full entry on visiting the Lord Byrons take a look in his journal by asking for MS-983, Box 2, Folder 1.