If you're having trouble fighting vs high level pokemon with magikarp you can still evolve, even if the required exp to level up will increase, but overall I'd say it's worth doing after level 70/80 :)
Keep in mind that for kanto e4 even a level 90 gyarados will be enough :)
I 1-shot everyone in Kanto with my gengar. But johto e4's mega beedrill is rocking me I can't get passed... I was thinking with skuntanksuckerpunching I might as well set up a dd gyarados like every1 suggests...