Jake's kids do show up from time to time, and I do feel that Jake has some unresolved feelings toward them as show in the Pillow dream episode and the last Grayable.
As for the ending, I actually liked how sweet it was. And come on, this is BMO we're talking about it. He accepted a marriage proposal from a bubble, believes his reflection is a girl named Football and seemed to be completely bonkers in BMO Noir. BMO nonchalantly saying hi to a city full of his relatives seems perfectly normal to me.