Haven’t you heard that David Bowie song, "Ch-ch-ch-changes!"? The "mutability" of the title of Wordsworth's poem is just a synonym for transience, or the capacity for things to change and pass away. Another synonym is "dissolution," which has more negative connotations because it deals with things coming to an end. But, yeah, it’s all just transience. Wordsworth is pretty divided on the subject. On the one hand he’s a real nature lover and has to respect that the processes of nature are right and appropriate. But he’s also human, so he gets emotionally attached to things and doesn’t like to see them go away. In the end, he shows a kind of quiet respect and awe for nature’s transience.
Wordsworth argues that there are no universal truths except those contained in general forms and processes. In other words, there cannot be specific ideas that remain true forever.
The image of melting frost differs from the image of a tower that falls because with the frost, the basic form and structure of the object does not change.