I have never heard of corn being an allergen either. However the question of whether it is a 'filler' becomes rather semantic as we debate what is meant by filler.
Many wonderful dog foods have corn as an ingredient, but I would stay away from any that have it as the first ingredient. The primary ingredient should be a good source of (digestible) protein. Most often now you'll see chicken, but lamb is becoming more common, and a few 'flashy' foods use duck, rabbit, or buffalo - while good these are a bit over the top. Sometimes if you pack in too much nutrition it just means there's plenty of nutrients left out in your yard.
But that is dog food, and dogs are omnivores. Cats, though they seem smaller and sweeter, are carnivores. They really need meat for their diet, and they need complete proteins and ample taurine which is lacking in plant sources. This is especially an important requirement for pregnant or nursing cats and kittens.