The short answer
Yes, Marajo Island is worth visiting if you value a destination with a strong sense of place. It combines open beaches, buffalo culture, food traditions, wetlands, mangroves, river movement, and a slower rhythm that feels different from many Brazil itineraries. For travelers who like places with texture, Marajo can feel more memorable than a generic beach stop.
The answer changes if you want a trip that requires almost no decisions. Marajo is planning-sensitive. You need to think about how to arrive from Belem, whether Soure or Salvaterra fits your route, which experiences deserve priority, and how seasonality affects comfort. Those decisions are manageable, but they should not be ignored.
