If by atmosphere you are referring to the general feeling and aesthetic of Wesleyan, then the Wesleyan campus is gorgeous and as chill as they come. The community is welcoming, and the students are unique, driven, and decidedly weird.
If by atmosphere you are referring to the surrounding area, however, the response is less positive: Middletown is quaint and has one of the cutest Main streets in the United States, but it is not the safest place, and has been described as anything from "depressing" to "worrisome," depending on the time of day and number of people with you. There are a lot of tasty restaurants, a Rite Aid, and a theater within walking distance, but you have to drive to get to a supermarket or a Starbucks. All in all, Middletown is not the ideal "college town," but it doesn't ruin the Wesleyan experience.