The sonar is more of an artistic choice. Using a regular bar made it trivial (I would solve it in ~1s on average) so the gameplay was less interesting. In the current state it does feel slow because the game lacks the speed upgrades that were planned. However this game is also made to be slow paced, chill and nostalgic, part of the fishing experience is just waiting there looking at the art and enjoying the music.
I do agree though that the sonar part of the bar makes it harder to understand how the fishing minigame works. I personally remember gameboy games being a bit like this, not explaining really what to do and leaving you to try things and understand how they worked but I understand that compared to the more nervous and instant-feedback design of modern games, this feels very dated.