General:
Neutral airs are very useful, they scoop up opponents from anywhere close by and put them right in front of you for FAirs. Kraggs Aerials in general are very good and it might sound counter intuitive against a Wrastor but I like keeping the fight in the air. Both Maypul and Wrastor have tools that keep opponents off the ground anyways.
You are never going to outrun Wrastor or Maypul, instead be patient with attacks and parries. I'm not saying camp out because that strategy sucks balls and is no fun for anybody, just you can't waste attacks you need a good accuracy. I find that if I can hit 6 parries a match and an accuracy of over 50% then it's normally a win. (Easier said then done I know!)
Wrastor
Not going to lie I find this the hardest match up for Kragg but it's certainly winnable. The most annoying move Wrastor has is the tornado, it comes out so fast and even ignoring the speed buff if a great opener for lots of lengthy heavy hit stun combos. But it is normally predictable so if you can jump it or even better parry it you normally have a window to punish a Wrastor who will normally be following through for a combo, Fairs and smashes kill Wrastor super early and if you spot a gap go for them as the risk reward is high.
Lots of Wrastors like to spam smash attacks and if they hit you, it hurts. But they have a monster cool down if they miss, so try to trick people into smashing and dodge them.
And finally I've found the best place to be against Wrastor is diagonal bellow them. Kragg has lots of tools for sending opponents high and Wrastor only has a few tools for getting opponents bellow and most of them are super risky and punishable.
On the flip side, the absolute worst place to be is above Wrastor, especially high on the stage, never pillar to the top as a simple tornado will kill you from pretty much no damage.
Maypul
The first 40-50-60% of your stock against Maypul will just be gone in an instant, Maypul is so fast at wracking up early damage and there is little you can do.
However it is not game over, Maypul has one giant weakness and that is lack of kill options. Kragg is so heavy that even the normal off the top kill is very hard for Maypul, especially if you have good DI. Leaving Maypul's only good kill options his smashes which if you know are coming are fairly easy to parry as they are very choreographed.
Along with aerials in general a good way to beat Maypul is to just try and slow the pace down as much as possible. Pillars, down special rocks. If given time to use them, use them as must Maypul's are upset with changes to rhythm.
Here is the easy part, Maypul is bad off stage, his recovery is bad and punishable. Even if he has you tethered. If you know the tether is coming, neutral air, or even better, jump off the stage too so when he teathers he still can't make it back safely. Kragg can recover from anywhere, Maypul does not have that luxury.
Other than that, just parries and Fsmash reads. Maypul is either super campy or super aggressive. A campy Maypul's attacks are very obvious when they happen because he HAS to get to you to hurt you whilst as a Kragg you have plenty of ranged options. Whilst aggresive Maypuls are going to be on the attack nonstop so even randomly hitting parry will give you a few.
Even after all that, both matchups are hard, for me it's just about trying to get a read on your opponent and countering.