My only "bad" wonder trades was around release when I just did it for fun in Santalune Forest, so all I got in return were Pokémon from the same area. Since then I've gotten lots of decent Pokémon and some pleasant surprises! I am currently IV breeding for Gale Wing Adamant Flethling, so I am sending out all the Gale Wing Adamant Fletchling with less than 3 perfect IVs.