This riff is a classic blues riff and has been ripped off a million times – most surprisingly by Girls Aloud.
The riff alternates between the bottom two strings and the top two strings. The easiest way to play this is to strum down with your thumb on the G and C strings and strum up with you index finger on the E and A strings.
Your site has turned into a great resource and provides me much enjoyment. If I might provide a suggestion, a series of blues riffs (like Muddy’s signature riff you’ve so helpfully provided) for the uke would be a big winner from my perspective.