It doesn’t accomplish the same thing but it might do the job you need it to do. Most formal diffuser panels are designed to scatter frequencies in a fairly uniform way, and many of them work from the midrange up. Properly placed, they will reduce some of the frequencies that your room are causing peaks on and helping with some phase cancellations you may be experiencing. The end result is a more pleasing experience resulting in you hearing more of the music.
A bookshelf may accomplish something similar as the incongruency will reduce uniform reflections that often cause problems, as long as the bookshelf is placed in generally a helpful spot in the room. That said, it will not scatter frequencies as uniformly, and the density of the shelf and books may also absorb lower frequencies, while the furniture portions of the bookshelf may reflect certain frequencies.
Will it do the job you want? Maybe, you’ll have to experiment. But a bookshelf filled with books and other things is typically better to have on first reflection points (side walls between the speaker and you) than having nothing there.