Contrary to Leo's beliefs, Dumpy was never a useless frog, it could perfectly understand the stories that Leo narrated to it since birth and it looked up to Leo like an idol.
Since it was a newborn frog with undeveloped vocal cords, it could not respond to Leo as calmly as it wanted to, however, it understood everything that Leo conveyed to it nonetheless.
From Dumpy's perspective, Leo, a.k.a his esteemed 'Lord Father', was a towering figure of strength, wisdom, and incredible valor.
Ever since the moment of his birth, when Lord Father had first spoken to him, Dumpy had listened attentively, his tiny frog heart swelling with admiration and awe that it had gotten such an amazing Lord Father.
Though his vocal cords were too underdeveloped to respond, Dumpy absorbed every word, every heroic tale, and every grand story that Lord Father narrated and felt happy to listen.