In improving your Mega Kanga spread, a great place to start is: how fast and how bulky do I want it to be? Are there specific benchmark speeds that I need it to achieve? What attacks are common and should be EVed to survive?
Let's start with the Speed stat. If you're running Jolly nature, then I would personally say it's best to run the regular 252 spread. Here are my reasons why:
- If you're running Jolly, you clearly want to use 252 EVs in Speed. Here you understand that you're going to speed tie with other Jolly Mega Kangaskhan. If you were wanting to outspeed a specific threat, it's best to start at a neutral speed. Max speed Jolly Smeargle that isn't Scarfed reaches a Speed of 139. So using a Jolly nature would require 56 EVs invested into Speed to outspeed Smeargle. Using an Adamant nature, one would have to invest 156 Speed EVs to achieve the same objective.
- You always want to boost your most important stat. With Jolly, beating other base 100 Speed Pokémon is of vital importance. If you use Adamant, you know you aren't in the competition for the Speed tie.