A:
It's kind of nonsense. McGregor is just making fun of Dustin's injury and generally trying to make him feel bad.
Karma... is complex. They mean a kind of spiritual punishment. You do a bad thing, the universe will return that to you later.
a "Paw" is what you call the foot of an animal, like a dog or cat. He's using that as a slang term for Dustin's foot. Some people call hands and feet "paws" for style, but he might also be doing it to insult him. "Paws" could be seen as childish, or saying he's an animal. I'm not sure if he means it as an insult or that's just his style of speaking.
Specifically, Conor and Dustin are rivals, and they "have beef" (they some hostility between them). Conor and Dustin fought in 2021 and Conor's left leg was injured in the fight. At the time, Dustin tweeted that he thought the injury was Karma for Conor's behavior. Now, Dustin's left foot is injured, so Conor is saying this is Karma for him injuring his left leg. "You hurt my leg and said it was karma, but no see this is your karma for hurting me." I think it's both a dig at Dustin for Dustin saying Conor's injury was Karma, and also saying "this is what you get for hurting me."
Admittedly, I don't know the situation between the two of them well enough to know if this serious hostility, or if this is playful banter (what we american's called "Trash talking").
A note on Karma: Karma is a concept from Hinduism, a religion from India. It's an idea of a spiritual ledger of good and bad deeds that will determine the outcome of your next life. A lot of Westerner's have borrowed this idea and use it to think of as a universal balancing system: if you do bad things, bad things will happen to you because you've built up "bad karma", and you will be rewarded for doing good things. This idea is common in western thinking and idioms, the idea that fate will snap back at people: "you'll get yours", "what goes around, comes around", etc. My understanding of Hindu Karma is that this is not how it works at all. So while the word is the same, typically when Westerners say "Karma", they may not be describing what it means to Hindus.