Make no mistake – reviews from outsiders are very valuable! These can take many forms, e.g. suggesting improvements, pointing out subtle bugs, flagging style issues, requesting additional docs or tests to be written, discussing the right API, debugging test failures, and more.
What’s not valuable is reviews from outsiders that simply say “LGTM”. At the very least you should say how thoroughly you have reviewed the code and what things you looked for (e.g. items from the above list).
If you simply want to express your desire to have the proposed feature or bugfix, use GitHub’s emoji support and add a heart or thumbs-up to the issue.