Adding to the point that he distances himself from the class, they would also be somewhat wary of him after the exam incident. So really it’s not that bad as some people think??? Also, they knew that he was suspended, and may have possibly won a trip around the world, whereas, with Tsumiki they didn’t have much information - no idea whether or not Imposter-chan had told them - and contrasting to Nagito they have known her for a year and have spent time with her - heck even Hiyoko was (somewhat) worried about Mikan. Additionally, the time at which he entered the classroom, they were all worried about Mikan - therefore they’d get their hopes up that it would be Mikan to enter, not Nagito. So it’s not really that surprising that they were disappointed with Nagito appearing instead of Mikan????