I don't think the intended audience is relevant. I still don't see what about it is particularly uninteresting. It's all diatonic, sure, but if that's the criterion then a huge amount of music is uninteresting. The melody has a variety of intervals (including an octave), and suspensions on nearly every chord.
I took the comment to mean that Happy Birthday to You is not interesting compared to, say, Locatelli's Sonata in G Major. The second is simply a more ambitious work, is all.
