Do I learn objective C? Is using Cocoa the easiest and best way to make a UI?
Cocoa is the best way to make a reasonably full featured app with a completely native behaving UI for a current Mac.
A reasonably skilled C++ programmer can pick up the basics of Objective C, syntax and semantic differences, in maybe 2 weeks of part-time study. It just takes a bit of time to get used to how Objective C's form of punctuation abuse is different from C++'s form of punctuation abuse. Learning the vocabulary of the basic set the Cocoa APIs and more in-depth Objective C coding techniques will likely take longer, depending on how much you need for your project(s).
Whether that's easy or not is relative.
Cocoa is definitely what Apple advises and supports. It's worth noting, however, that while the bits and pieces of code that actually connect to Cocoa have to be written in Objective-C, the rest of it (that does that real work, for example) can all be written in "Objective-C++", which is just normal C++, with the extension changed (to ".mm", IIRC).
While I can't say I've ever taken any kind of a liking to Objective-C, I've written a few Mac apps, and never really had any problem with using it where needed.
If you want to stick with C++, take a look at Qt, which is a cross-platform C++ application framework. It's used to build the K Desktop Environment (KDE), and is used in commercial cross-platform software, such as Adobe's Photoshop Elements.