I believe mutability is a trade off. You can make objects more immutable by being less efficient in terms of storage size, search time, source code simplicity. In exchange for these negative attributes you get source code you can proof more easily. It's good when that attribute is needed, but not most of the time.
Git is only immutable on that meta level where we talk about the object key-value store. The implementation uses mutable files though, and even steps away from that immutable key-value store when the size gets too big and needs to store more efficiently.
And on the user level you can amend, edit, squash and delete commits easily, which is actually a strength of git not a weakness. People love it for being more mutable than SVN.
