This app was mentioned in 4 comments, with an average of 2.25 upvotes
Yep, I started keeping track since the money/fitness splatfest in EU, mostly I just note whether I won or lost, what map it was on, and my K/D ratio. And also noted any disconnects I had. It's more for a bit of fun than anything, I like having my bare stats from splatfest to splatfest. :3
I know there's an app for the android called IkaReko, which I believe you can use to manually record your stats as well, it's in Japanese though.
Yup – anything can use stat.ink's API to upload battles – IkaLog just does it automatically based on game event/video recognition. IkaRec works quite well, but it's in Japanese (and not open source & no plans for localization – I asked): https://play.google.com/store/apps/details?id=com.syanari.merluza.ikarec&hl=ja
This is a ridiculously amazing and complex site by a Japanese user fetus-hina. It uses screen recording software called IkaLog that, if you have your Wii U hooked up through your computer, can analyze battles and provide detailed stats (example) of when in a match you died, by what means, number of kills/deaths, stats of all players on both teams, etc. It also tracks changes in rank, money, gear, etc. The code's all on GitHub as well, for stat.ink as well as the cross-platform IkaLog software.
I do manual data entry via a (Japanese only for now) Android app called イカレコ (IkaReco, SquidRec) where I can enter the battle type/rules, map, K/D, and points scored. It works with stat.ink via an API key provided by the site and uploads your data (I just started using it, obviously).
Then there's the sister site https://fest.ink/, which provides detailed stats about Splatfests and shows global short and long-term winning percentages (example, of the Squid vs. Octopus JP Splatfest). fest.ink does not have an English-language interface, but stat.ink does, accessible through the menu bar at the top.
Here's a demo video of IkaLog (bad, but all I could find): https://www.youtube.com/watch?v=aYBYpwjk2F4
I found this link on their Twitter page: https://play.google.com/store/apps/details?id=com.syanari.merluza.ikarec
For iOS there is an alternative app named Ika Kiroku.