They weren't.
First, only Paramecia and Logia would fall short; three is the magical number and a trio tends to work better than a duo. That aside, animals are widely liked (and for an artist it gives you more to work than just designing humans), they provide natural hierarchies to work with (carnivorous versus herbivorous, current < prehistoric < mythical, etc.) and, on top of it, there's a whole plot regarding devil fruits and experimentation on their nature to be explored through the Zoan type (and, in my opinion, the concept of artificial zoans, object with zoans, Vegapunk's research on animal cyborgs...) which in my opinion is, by far, the most interesting thing Oda has done with fruits.
So no, they weren't a mistake. They are needed for many reasons, including a whole plot on what devil fruits are and how they work. If anything, I'll agree that Oda could have made a better use of them, but the concept itself is good.