Fast note app for quick lookup & copy to clipboard for osx & ios?
Before searching for a 3rd party utility, I would recommend trying out Apple's built-in Notes app. It seems to satisfy your three requirements as you can copy/paste from the clipboard, view them on both macOS and iOS, and it syncs with iCloud across all your devices (plus iCloud website).

Checkout these links for a how-to guide on each of the listed platforms:


iOS: Use Notes on your iPhone, iPad, and iPod touch
macOS: Notes for Mac: Notes overview
iCloud: iCloud: View notes on iCloud.com