Good Big Brothers Propaganda!!
The fact that "guess who's going to be up all night again" has Donnie looking all worried. They aren't even mad that he's reading instead of focusing. They're concerned about his sleep schedule and the fact that he's doing things harmful to it (and therefore himself). He's probably gone to them in fear before, if this is any indication of Mikey having a history/pattern of reading horror comics with similar results.
Raph even lets Mikey think he got away with his terrible lie about cleaning his grappling hook to save face.
"Seriously, Mikey. What are those horror comics good for other than freaking you out and-"
Raph's saying it in a harsh way, but the communication is "stop doing something that's going to scare you and hurt you in the long run," and Donnie's face perfectly encapsulates that. The middle brothers ever.
