Of course you can learn plenty of things from manga! Reading is always an educational journey, and though comic books employ more drawings vis a vis words, it is a visually aided education nonetheless. From something as rich and action-filled as the history-based romantic story of Rurouni Kenshin, you can pick up nuts of knowledge such as the different ways a sword can maim you, names of Hiten Mitsurugi-ryu techniques, and specialty sweets in Kyoto.
I’ve learned the following things too: