I noticed yesterday that there was a onyomi tag, but not a kunyomi tag. Is there a reason for that, or is that just an oversight?
If it is on purpose, can someone help me understand why? I'd be interested to find out.
I think the onyomi tag is primarily used on questions that involve different types of on'yomi. (I would consider Did I determine the pronunciation of 人口 correctly? a typical question for this tag.)
This is also mentioned in the tag wiki of onyomi:
音読み. A Sino-Japanese reading of kanji. Usually divided into 呉音 "Wu sound", 漢音 "Han sound", 唐音 "Tang sound", and 慣用音; the first three for readings imported from the corresponding Chinese dynasties, the last for exceptional readings.
There are no such ambiguities for kun'yomi. Questions about kun'yomi are usually about etymology and thus tagged etymology.
The tagging system is ever evolving and improvements are always welcome. If you think that a tag (for example kunyomi) might be useful, you can start a meta discussion in which you explain why it is a useful tag and find at least a handful of questions that would fit this tag.