There's no simple answer to this question. Certainly IMO just swinging a bat is much easier to learn than bowling, which isn't a natural motion as all. But beyond that I don't think it's hard to learn to bowl seam up or basic finger spin, whereas even competent defence requires a bit more application. Hence why the bottom grade teams (like the ones I played in) often seen to have eight okay-ish bowlers and two batsmen. But as you get to higher levels pace bowling in particular becomes much harder. It's as if starting from a level of competence batting 'scales up' faster than bowling.
Part of the issue I think is that the learning process is different, and that becomes more important the higher the level. As far as I can work out even top batsmen are nothing special in terms of physical attributes like reaction times, though obviously some base level of coordination is needed. Batting relies on acquired instinct, where the batsman has learned to subconsciously judge where the ball is going early and move accordingly. Why some can do it and not others, I can't say.
Pace bowling, on the other hand, absolutely requires certain genetic traits imbuing coordination and fast twitch muscles, hence some bowlers even with loads of practice and perfect technique will never be as fast as others. On the other hand it, IMO, seems to involve less learning - those who can bowl quickly and rhythmically seem to be able to put it all together very quickly (obviously some improvements are possible after that).
I would say spin bowling, or at least finger spin, is the most learnable. You don't need the physical traits for pace bowling and the time and repetition to develop the instincts for batting. But even then there is variation - it never ceases to stagger me how much faster professional spin bowlers bowl than amateur, and doing that takes effort.