I have mixed feelings about this, but I lean towards keeping the status quo.
I feel that our community is already somewhat unwelcoming with all the rules and restrictions. And I don't think we should add extra constraints.
Besides, the Ubuntu Code of Conduct and the SE Code of Conduct align well. If anything, I feel the SE CoC is more "restrictive" and covers more conduct-related behaviour than the Ubuntu CoC.
The extra items covered in the Ubuntu CoC are more related to teamwork and leadership in projects, Which is not something needed by newcomers asking or answering questions.
However, moderators in Ask Ubuntu have an active leadership role, so asking moderators to sign the Ubuntu CoC is relevant.
Ps. Just to make it clearer. I'm not against the Ubuntu CoC. I've signed it a long time ago and I completely agree with its content. I just don't feel we should force it on every user in Ask Ubuntu especially since the SE CoC is already enforced.