Unfortunately for us, Lightroom Mobile doesn't have metadata features yet. No keywording, no access to metadata (including captions and titles), etc. We can only hope these types of features will come in a future release.
Feel free to voice your request in the wish form:
Thanks. My husband Joe has discovered a couple of strange things regarding this issue. There is a setting in Lightroom mobile that allows you to "Share Metadata" and choose "captions". But even when we put captions rather than titles on our LR photos, it didn't show using the LR mobile app. However, when we chose to view photos on the website, the captions show up. An obvious bug.