The minor background characters on the cover certainly resemble Bert and Ernie. Those two characters were probably created as a homage to the famous Sesame Street duo, and they are clearly "Bert and Ernie"-types. But legally they are not Bert and Ernie from Sesame Street; and BOOM! isn't going to market these books with a big "Featuring Bert and Ernie from Sesame Street" sticker on the cover or use the trademarked names and logos or major uses of such characters (unless they get permission for the various rights).
It's the same principle that allows "The Simpsons" or "Family Guy" to have characters that resemble the Muppets or Sesame Street characters appear on their programs. Legally these guys aren't the real Bert and Ernie of Sesame Street. The only way they could get in trouble for these "Bert and Ernie"-like characters is if Sesame Workshop can prove that the characters in these comics infringe on their use of the characters and harm the integrity of the real characters, or if BOOM's use of such character can be proven to be misleading consumers and negatively impacting Sesame's brand and/or harming their business. A little homage or parody like this is legally fine.