This mod would help to modify the difficulty of fishing. It simply multiply a ratio to the origin fishing data. The difficulty is 80% of origin one by default, you can change it in config.json in the mod folder.

For StardewValley v1.11 and SMAPI v1.6+

If you have issues, double-check that you have the lastest version of SMAPI.

Source: https://github.com/hcoona/StardewValleyMods
Bug Report: https://github.com/hcoona/StardewValleyMods/issues