More than mistake, the most appropriate words are "wasted potential".
Surely Palpitoad isn't treated very well (a trait shared by his pond friend, Stunfisk), but at least he is treated better than Kingler. He needs some development, but who knows if the writers will give him what he deserves. At this point, I wouldn't be really surprised if he becomes a mix between Kingler (powerful Water-type) and Croagunk (three moves syndrome and lack of evolution). Hopefully the second part doesn't happen.