A patch doesn't necessarily imply a bug fix or an improvement which everybody may benefit from. I mean, in many cases one may just want to run a custom patched version of some software (e.g. dwm comes to mind, as it's a common use case), apply some unofficial community change or even test new features (plugins , etc..) , committed to mainstream but maybe not yet merged to the main branch. Those things are not fit for the pkgsrc tree: they may not meet stability/quality requirements, and be regarded as unneeded by many users. The sidebar patch/plugin for mutt is a perfect example: it's not part of mutt's mainstream, but has been included in neomutt. As I do not need neomutt and can happily leave without its dependencies, but I do like the sidebar feature, I just stick to a local patch (keeping in mind I won't expect help support for this feature). I think that is what LOCALPATCHES is meant for.
FVWM has seen many community-contributed patches over its long history, and OP may just want to test some unofficial add-on nobody else is interested in, rather than fix a bug or improve the package's quality, reliability...
If that's not the case, then getting used to edit patches like they're meant to be committed to the upstream pkgsrc repo, is probably the best, most advisable and definitely most useful approach.