Hmmm...Adamant Mega Kangaskhan already outspeeds Adamant Lucario, and regular Kangaskhan speed ties with it. Without Adamant, you lose out on some pretty significant KOs (the most significant of which that I found was the OHKO on Hippowdon at +2, and the 2HKO on Aegilsash at +0 using Bulldoze), so I'm not totally convinced that Jolly should be the main slash. I'm not completely opposed to slashing it after Adamant, but simply outspeeding Lucario, Haxorus, and Hydreigon in normal forme isn't enough in my eyes, especially since Pokémon below base 100 Speed tend to run neutral natures.
Adamant should definitely be slashed first in my opinion, though there are some advantages of running Jolly. You already mentioned Hydreigon, Haxorus and Lucario. What I'd like to add is offensive Gliscor (very rarely used), opposing Mega-Kangaskhan, since Kangaskhan is one of the few offensive Pokémon that can actually live a +2 Sucker Punch without resisting it, even after Stealth Rocks. Mega-Charizard X and Mega-Charizard Y can outspeed and OHKO you without going down to a sucker punch, though if you're at +2 and have Stealth Rocks down there is no chance of them living it. Mega-Guardevoir and Mega-Medicham although I've only encountered them only once or twice, are also a thing. Mega-Garchomp is one of the best checks to Adamant Mega-Kangaskhan in my opinion. It can outspeed and hurt it with double Rough Skin before Mega-evolving and has great bulk for an offensive Pokémon after. It always outspeeds Adamant Mega-Kangskhan if it is Jolly. In general, speed tieing with other base 100's can prove to be extremely useful, as for example you won't have to predict around Roost Volcarona or Dragon Dance Salamence with your Sucker Punch. Kyurem and Hihidaruma are two Pokémon you very rarely encounter, but they do have a spot on hail and sun teams respectively.
Rocky helmet garchomp or ferrothorn should be mentioned as checks/counters. Really rocky helmet in General but especially those two, as a contact move from kanga does 58?% to itself. It sounds gimmicky, but my rocky helmet chomp is my main way of dealing with it. Also will o wisp gengar destroys it once it mega evolves. The fact that I'm using things like this shows how ridiculous mega kanga is
I disagree with Rocky-Helmet Garchomp, as it just gets wrecked by Earthquake. If you are using Mega-Kangaskhan you'll probably be sending it out very late in the game to get off a sweep, meaning you're already aware of your opponents sets, especially because Rocky Helmet Garchomp is more of a early- and midgame pivot.
+1 252+ Atk Kangaskhan Earthquake vs. 4 HP / 0 Def Garchomp: 182-215 (50.83 - 60.05%) -- guaranteed 2HKO after Stealth Rock
Unboosted Earthquake is an easy 2HKO and you take zero damage since it doesn't make contact. If Chomp chooses to run bulk then it will either have to lower it's speed or attack stat, meaning it will take on Mega-Kangaskhan worse than it did before. It is also highly unlikely that Garchomp won't have taken prior damage. Hazards and stray weak physical moves will definitely be wearing it down to where it cannot switch into Mega-Kangaskhan anymore.
Ferrothorn, on the other hand is just a straight up counter unless you are running Fire Punch or it is severely weakened. Unlike Garchomp though, Ferrothorn has recovery in Leech Seed and potentially Protect.
+3 252+ Atk Kangaskhan Bulldoze vs. 252 HP / 48+ Def Ferrothorn: 121-143 (34.37 - 40.62%) -- 99.51% chance to 3HKO after Stealth Rock
+3 252+ Atk Kangaskhan Earthquake vs. 252 HP / 48+ Def Ferrothorn: 201-237 (57.1 - 67.32%) -- guaranteed 2HKO after Stealth Rock
Only Adamant Mega-Kangaskhan after a boost will be able to beat Ferrothorn if it is still healthy. You cannot boost on Ferrothorn either because the residual damage will absolutely destroy your life.
