There are tons of threads requesting this feature, one thread has 217 upvotes since 2014. The contributor that submitted this idea, @johnrellis, wrote a plugin called Any Comment to do just this, but it is no longer compatible with LR Classic v13.3 or higher. Attached is a screenshot of how John's Any Comment plugin allowed me to write the likes and comments received from LR Web/Mobile to the metadata field of my choice in LR Classic. This allowed me to filter those comments for a specific word.
Adobe, please consider adding this feature for those of us with workflows that involve team collaboration to review and comment on thousands of photos that are synced to Lightroom Mobile for review and commenting. Without this, I now have to manually add the comments from my team to the LR Classic Metadata, just so I can filter for them.