Mocking

Certainly she marries for security without affection but I wouldn’t go so far as to say she mocks him.

Re: Charlotte freely admits spending as little time as possible in contact with Mr Collins

I may have missed something but I think most of that is Lizzy’s observation, Charlotte admits to encouraging him to garden but she never says it's for the purpose of avoiding him. I don’t take that as mocking. Looking over that passage again:

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To work in his garden was one of his most respectable pleasures; Elizabeth admired the command of countenance with which Charlotte talked of the healthfulness of the exercise, and owned she encouraged it as much as possible.

I guess Ican see how one could take that as mocking. A lot depends on her tone here. I’ve always pictured it as Charlotte speaking sincerely and Lizzy seeing through her.

As far as her being embarrassed by him:

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[Elizabeth] looked with wonder at her friend that she could have so cheerful an air with such a companion. When Mr. Collins said anything of which his wife might reasonably be ashamed, which certainly was not unseldom, she involuntarily turned her eye on Charlotte. Once or twice she could discern a faint blush; but in general Charlotte wisely did not hear.

So out of all the times Lizzy would expect her to be embarrassed she only discerns a faint blush once or twice and she is specifically looking for it. Embarrassment is hardly mocking and she barely shows it. Plus all this is in the context of Mr. Collins showing off the house and “addressing himself particularly to [Elizabeth], as if wishing to make her feel what she had lost in refusing him.” I think most people would embarrassed if their husband were showing the person they proposed to first all they had missed out on.