The adipose are not restricted from learning martial arts and frankly, physical fitness of a high degree is not required to turn someones lights out and fitness doesn't translate out as martial ability or skill either.
I've seen obese people clock young healthy and strong guys with ease.
I've seen fat guys press fit guys against a wall and smash their lights out.
If you get all caught up in what you look like, then go do bodybuilding or something.
You should be as fit as your ability requires you to be when it comes to martial arts.
If you are into competitive mma, then you will not do well if you are not keeping your fitness up.
If you are learning how to cut someone with a machete, I don't see the point of how fitness is going to make you better at that. lol
don't get me wrong, I support being fit and healthy and good diet and all that, but I have also been exposed to enough now that I know that martial arts skill and ability frequently has nothing to do with an ideal of fitness that is given to us by the Tony Hortons of the world.