Possibly the most famous/infamous Kangaroo cover of the Golden Age is from Choice Comics #1 (December, 1941). It cover star goes by the name of Kangaroo Man, and he seems to leave most of the dirty work to his pet kangaroo, Bingo. I really like this cover as it has that great Golden Age 'fresh out of anatomy class' look to it. The artist's name is Chuck Winters. I do not know a thing about him, but I do like how he channels Lou Fine here - as this cover is jam packed with sinewy muscles. Bingo would show up again on the cover to the second issue before fading (hopping?) into obscurity.
I am too young for Mutt & Jeff to really resonate with me, but they a place in pop culture history, as they appeared on the first cover to Famous Funnies #1 and their their comic book strip lasted nearly 80 years. They had a long running comic book series published by DC, Dell and then Harvey. Mutt and Jeff #113 (August, 1959) was the third from last issue of their Dell tenure, and it features a fun Kangaroo gag by artist Al Smith. There would be two additional Kangaroo themed covers during the Harvey years, so as far as I can tell, Mutt and Jeff hold the record for Kangaroo covers.
Disney based comics seem like a good place to find Kangaroo covers, and I can think of at least two. My favourite is Donald Duck #92 (January, 1965) with Donald as the Kangaroo Kid. His nephews look as though they are having a blast hopping along in the kangaroo's pouch. The diligent work of Alberto Becattini informs me that this story has been reprinted like crazy, with the cover being recycled for Donald Duck #188 in 1977. I believe there is at least one Mickey Mouse cover with a kangaroo and, of course, Kanga and Roo show up on quite a few Winnie the Pooh covers.
I have saved my favourite Kangaroo cover for last. Dale Evans #23 ( May-June, 1952) is an absolute riot. Although the series features mainly photo covers, Jim McArdle tackled the art chores for the final 10 issues and somehow fit in an ostrich, an elephant and some tigers to go along with this kangaroo. I can believe anyone would ever accuse DC of being boring in the 50s. This is a fun story, in which Terrific Toby and his kangaroo, Caperin' Kate appear at the County Fair. Toby and Kate help Dale hunt down some outlaws who stole the Fair's prize money. It's a great story with the kangaroo's pouch playing a vital role. Good stuff.