The Davy Back Fight is not useless, nor is it a piece of skippable filler. In fact, it's one of the most enjoyable arcs in the entire show, because it has a very specific jobs to do, and it does them very, very well.

Is it okay to skip Foxy arc?

If you're watching the anime, watch it the first time through. Maybe look up which episodes of it were filler and skip them if you're having a hard time with it. Once you've seen it once, skip it from then on. Never skip anything.

What was the point of Davy Back Fight?

The Davy Back Fight is a traditional pirate game held in homage to the mythical Davy Jones, meant to challenge—and conscript—members of opposing crews. Canonically, only the Foxy Pirates have been known to initiate these games.