Learning to read sheet music at the conceptual level is not that hard, but memorizing every note takes some effort. Most ukulele music provides tabs, but many books provide the melody of a song in traditional notation only. Take the ubiquitous Daily Ukulele – if you don’t know the melody of a song in that book, traditional notation is your friend.
Here’s an excellent practice tool to help you hone your site reading chops. It’s hard enough to remember where every note is on the fretboard, so not having to think for even a second about what the note is you’re looking for goes a long way.
The musictheory.net site has tons of great tools, but this one is worth highlighting. It shows you a note, and you use your computer keyboard or mouse to name it. For speed, use the keyboard. Hold down the Up arrow key for sharp notes and the Down arrow key for flat notes.
The link above is a customized version of this tool for ukulele players. I turned off “accidentals” to eliminate the sharp/flat complexity and removed the notes below middle C (you can undo this if you use low G tuning).