Magikarp

That depends on two things
(1) do you lack stronger water pokemon (i.e. kyogres) in your current line-up
(2) do you want to show off your shiny pokemon in raids/gyms

If the answer to either of those questions is yes then I would say go ahead and power it up.

It's a Shiny and with good IV's, so yep! Power it up.. when you get some Candies for it.

But on a more logical side, a few questions:

1) What are the A/D/S stats on it? 88% can be good, but it can also be a 10/15/15, which may not be the best option to power up.
2) How often do you find Magikarp in your area? If you can find on that's Weather Boosted and has Attack 15, that might be more worthwhile.
3) Do you have "Ultra Friends" you can trade a higher level Gyrados with? It may be possible to trade with someone and get a higher level Gyarados (and maybe a Lucky Gyarados) and save some costs on the powering up.

Still, tho, 88% Shiny Gyarados - I'd be powering it up just cuz'.

No-brainer. Power it up. I'd even say max it, but you can take your time with that. It's a shiny with great IV's and the best moveset. There is absolutely no good reason not to power it up.