All of the changelings, Odo included, were capable of extremely sophisticated shapeshifting, so the excuse that he never got the hang of humans was frankly bollocks.
That always bugged me. He can turn into a bird, capable of flight, which means rather complex feather patterns. Heck, his hair is more complex than his face. And he can do the double-nostril thing like the Cardassians
The only real explanation I can think of is that he practices his shape-shifting by surrounding other objects - so maybe he never just slimed anyone's face before