When I was there in June, I saw the Princesses (including Ariel) doing scheduled autograph signings at the theater next to the Castle. See this thread for photos and more info:
We met Ariel at the Princess character lunch at Auberge de Cendrillon but its luck of the drawer which Princesses are there so Ariel may not be.
We also met her at a meet and greet the Princesses were doing near Bella Notte at Easter- it was a free for all- no orderly lines and was not advertised as a meet and greet we just came across them by chance. We didnt notice any characters there doing meet and greet this summer.
You could always call into City Hall to ask if/where she may be available otherwise the parade is probably your best bet.
we met Ariel and Eric when we went for dinner at Auberge de Cendrillon - she is also my daughter's favorite princess but its a hit and miss as you don't know which princess will be in the restaurant and when.