I have to pretty much agree with everything SpinneyBigBird1 said.
It's a tough call for myself, since I consider Joey one of the most talented writers SS has had. My biased opinion and feelings on Big Bird aside, one could argue since Joey took over as HW that the writing in the show has actually gotten better, at least in terms of parodies and staying current. However, giving him head writing position also exposed his biggest weakness, which apparently was having trouble writing for certain characters, such as Big Bird. Not excusing this, since as SpinneyBigBird1 pointed out that there ARE other writers - and if it's because Carroll's aging, there's Matt, so those points are non-excusable.
But could it also be budget? Think about it. Big Bird isn't the only character who hasn't appeared much. It seems the only characters who appear regularly now are one's with skits (Elmo, Abby, Grover, now Cookie) and street scenes with Rosita, Telly and Baby Bear. And of course Murray. Lack of Muppet performers availability also plays a huge factor (Steve Whitmire anyone? Even though I'm aware for some reason new B&E skits DO exist but haven't aired yet. Thanks SW.
).
While I'd love for the show to have more of an ensemble feeling then anything, this sadly is just a sign of the times. Sesame Street is always changing, and even if it isn't the show I or you grew up with, as long as today's generation are enjoying the show and still getting the message, what more could we ask for? And who's to say the Bird (or X character you feel has decreased airtime) won't resurrect in popularity in the near future? Many characters have a dry spell (seems Grover did in the 90's, now look at him) but bounce back. And besides, SS would NEVER get rid of Big Bird, so no worries.
The Bird will be back.
...Just give him more freakin' scripts you darn writers!