A pair of sunglasses next to a friend's name in Snapchat indicates their participation in Snapstreaks and their return of at least one of your snapshots during the previous day. It highlights their activity.
On Snapchat, sunglasses emoji () next to a friend's name means that you share a mutual best friend. This indicates that one of your best friends is one of his one. This does not mean that you are the best friends directly, but both of you often interact with the same person on Snapchat.
The sunglasses emoji () on Snapchat means that you and your friend have a mutual best friend. This suggests that both of you frequently interact with the same person. While it shows a shared connection, it doesn’t necessarily mean you chat with each other often on Snapchat.