I don't think it has anything to do with the art per se. I mean, why should the fact that you're on an "aikido journey", so-called, be the reason why you question whether or not it is appropriate to smoke marijuana? The question makes no sense to me. It's like asking, "Should you eat pork rinds if you're a CPA?"
I agree. Or "should you drink whiskey..." It all depends on what you want from it I suppose.
I'm guessing the OP thinks of smoking marijuana as at least a quasi-spiritual experience and also considers Aikido to be and of course is then wondering if the two somehow are inapropriate together. In the sense that smoking anything is bad for you, I'm guessing it's not ideal. As for the mind-altering or potentially spiritual quality associated with it I can't guess (though I agree with everyone with regard to being on the mat: it's not appropriate). Personally I think the ability to induce natural highs is far more useful. Not only is it a LOT cheaper, but the ability to induce your own "zone" or "trance" is a lot more self-sufficient than relying on some external substance. Personally I look to practices like Aikido to teach me better self-control and self-awareness and the best approach for me has come from removing things like marijuana from the process. I used to enjoy taking a toke and swinging the bokken around, but any of the small leaps I may have ever made in Aikido came during times when I was not high.