Mediocre by what metric? American STEM education seems to objectively be doing pretty well in terms of Nobel prizes, scholarly journal articles, patents, technology product revenue, etc. Of course there's always room for improvement.
Like a sorting algorithm which is O(n) on nearly-sorted input - the utility is limited.
I hate to break it to you, but a lot of our most valuable research is produced by people who did their primary education outside the US. Just go to a STEM research lab at any US university connected to a Nobel prize or Fields medal in the last 10-20 years, and it will be almost completely made up of internationally educated students / professors / etc.