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.
Um, legally Bert and Ernie, and all Sesame Street characters are trademarked as Muppets.
Also, Bert and Ernie are clearly in the pilot bonus feature on the Muppet Show seasons.
I get what you're saying, but how sad is it that a Muppet property can only "homage" other Muppets?
I guess in a way it's like how Matt Groening said Futurama and Simpsons(in a 1999 interview) would crossover cameo now and then, especially with Kudos and Kang. But then the only "crossover" we get is Fry noticing a pile of Bart dolls and then a "surreal" sequence of Bender in The Simpsons.
Now legal mumbo jumbo aside, it *is* Bert and Ernie on the cover, who *are* Muppets.
Just because Jim Henson Company for example, can't refer to Fraggle Rock as Muppets...they're still Muppets. Jim Henson Company is allegedly not the owner of the Muppets anymore, but they routinely(thank goodness) use Muppet puppets from a variet of *Muppet* franchises and specials as characters in their Puppet Up and other HA projects.
Bottom line, only in legal mumbo jumbo world does that stuff even matter. Because to most fans including myself; The Muppet Show, pre Muppet Show specials, Sam and Friends, Bear in the Big Blue House, Fraggle Rock, Sesame Street, etc is and will always be Muppets(tm)