I'd say the admin app already looks a bit like Bootstrap, and it's hard to claim either in its default form is more or less "homely" than the other. Granted, I like both.

Also, the admin app isn't supposed to be client-facing; it's a very effective but not-extremely-user-friendly way to view data without having to resort to SSH'ing in and running ORM queries on Django's CLI (or, worse, the database's CLI).

You can use it that way (and many of us have; I'm not innocent), but encouraging that with a pretty application of Bootstrap will likely give a lot of new developers the wrong idea on the tool's purpose.