Thoughts on my fishing mechanic?

First, I disagree with the name "fish up". Fish is already a creature type, and as this ability doesn't necessarily go for fish, which might cause confusion. I'd rather go for something like "Catch" instead. That also is less limited flavorwise to fishermen, and could also be a hunter or beasttamer thing.

Second, you don't actually need to explain the facedown pile thing or top card. A good example of how to word this might be: Ghoulraiser. "Return a [cardtype] at random from your graveyard to your hand." This also takes away something else that isn't right in the current wording: what do you do after with that facedown pile? In modern magic that isn't a big problem with players thinking up what happens, as you shove it back in somewhere in the graveyard. But older formats, there's actually the thing that you have to keep the graveyards ordered for cards like Barrow Ghoul. The "random" wording allows players in those settings to keep the grave ordered, but simply hit a random number generator or a high enough dice to get a random card.

Besides those, I do think getting random things from your grave hasn't been explored enough yet as a design space, so this fills that up with something that does sound fun.