Many girls might have been taken in, for never were such attentions; but I knew the fickle sex too well.

Well, it seems fairly obvious that Isabella *was* thoroughly taken in by Captain Tilney, at least for a while (or she would never have broken off the engagement with James Morland), and this is an attempt to recover her dignity by going back to her former woman-of-the-world persona. ;-)

I pitied his taste, but took no notice of him.

Captain Tilney is now clearly hanging around this other girl (or was, when Isabella wrote her letter!), and I picture Isabella gnashing her teeth, visibly not speaking to him, sending him death glares whenever they found themselves together, perhaps stalking dramatically away, and generally complaining to her family about the "nasty little freckled thing" who had replaced her in his affections.

IMO, one part of Isabella's letter that is quite sincere is the opening:

...this horrid place... [snip] Thank God, we leave this vile place tomorrow. Since you went away, I have had no pleasure in it — the dust is beyond anything; and everybody one cares for is gone.