Despite their reputation, some mainstream newspapers have been accused of spreading misinformation.

mainstream 是由 main（主要的）和 stream（流）组成的复合词，字面意思是"主流"，比喻指"被大多数人接受或认可的主流思想、文化或媒体"。在句中指"被大多数读者认可的主要报纸"。