Yeah, fishing can be tricky to get the hang of, and some of the fish in the game are dang hard to catch. There's no direct relationship per se, as the difficulty per type of fish is a set amount, and on a scale completely separate from the player skill of 1-10. But as it relates to your question, as you level up your fishing skill, the size of the target bar will increase. There are other benefits too, like less energy consumption per cast, increased cast distance, less time before fish bite, and increased size of caught fish, but the bigger bar is the only one that will actually make the catching part easier.
As far as XP goes, per the wiki:
The amount of experience earned for each caught fish is as follows:
[1]
XP = (Fish Quality + 1) * 3 + (Fish Difficulty / 3)
Fish Quality = 0 for normal, 1 for silver, 2 for gold, or 4 for iridium
and also:
XP is multiplied by 2.2 if you also catch a treasure chest, by 2.4 if the catch was "perfect" and by 5 if the catch was a Legendary fish.
So base XP would be the amount youd get from catching a normal quality fish with none of the other multipliers. I assume having the fish difficulty on a scale of 5-110 is a way to get XP to work with the equation and keep the numbers on par with the other skills.