Don’t forget that -mainline kernels are completely unmaintained kernels without any QA or security reviews, they are built by some script when a new upstream tag is added, inside an unsupported PPA …
To have something like a metapackage you’d have to actually move the debs to the actual archive (a metapackage can not depend on something outside here) which would also mean the kernel team would have to do QA, security reviews etc. Do SRUs so the archive isnt stuck at some snapshot mainline version after release etc etc … this is a hell lot of work (my guess would be probably one or two fulltime devs just to keep this rolling … ).
So my guess would be that a metapackage inside the archive is very unlikely to happen …
Now … you could put a metapackage into the PPA but that would also need quite some manual maintenance to regulary bump the version, monitor when upstreams tags get updated etc … while this is likely a lot less effort than the first option, it is still non-zero and will cost developer time …
Lets take a look what the actual purpose of these mainline builds is at all (from https://wiki.ubuntu.com/Kernel/MainlineBuilds):
By default, Ubuntu systems run with the Ubuntu kernels provided by the Ubuntu
repositories. However it is handy to be able to test with unmodified upstream kernels to
help locate problems in Ubuntu kernel patches, or to confirm that upstream has fixed a
specific issue. To this end we now offer select upstream kernel builds. These kernels are
made from unmodified kernel source but using the Ubuntu kernel configuration files.
These are then packaged as Ubuntu .deb files for simple installation, saving you the time
of compiling kernels, and debugging build issues.
These kernels are not supported and are not appropriate for production use.
In that light i doubt that anyone would put any effort into maintenance work for these kernels… nor would anyone put a tool into a default install that installs kernels that are “not supported and are not appropriate for production use” …
That said, i guess nobody would complain if someone stepped up to put a tool like UKUU into universe (as long as it pops up something like the above warning text when you use it to not have illiterate users shoot temselves in the foot indeed).