For the sake of formality, let me first quote AskUbuntu Help Center:
Questions that you may ask:
Using and administering official Ubuntu flavors including: Edubuntu (14.04), Ubuntu GNOME, Kubuntu, Ubuntu Kylin, Lubuntu, Mythbuntu
(16.04), Ubuntu Studio, Xubuntu, Ubuntu MATE, Ubuntu Touch, Ubuntu
Budgie (17.04+) and Ubuntu "Snappy" Core.
Running third-party applications on Ubuntu.
Development on Ubuntu.
- Services provided by Ubuntu and Canonical.
Back to our discussion here. In accordance with the quote above, installing/using a different kernel is not off-topic for the following reasons:
so long as OP uses any of the official (!) Ubuntu flavors, they are supported, even if they're using a non-standard software. We have tons of questions that are not off-topic where users have installed 3rd party apps, 3rd party themes, 3rd party desktops. This leads me to another point below
Linux kernel is integral part of the Ubuntu system, and it can be considered under using and administering the system, but also potentially under running third-party applications. Even if you argue that a kernel wasn't provided by official Ubuntu repositories, it's still considered on-topic as running third-party application on Ubuntu ( even though kernel isn't an application ). So unless pigs fly, and Ubuntu somehow can run on top of BSD or Windows kernel, we still consider an Ubuntu with any Linux kernel on-topic here ( doesn't guarantee that there will be people able to answer that question, though ).