Randomness is anti-competitive only to a point. The ability to adapt quickly to uncertain situations is one of the key qualities in being a good smash player. I agree that items have potential for competitive play, but I doubt they will be widely adopted because the randomness and individual skill required by the player is extremely skewed.
If a Hammer spawns next to your opponent, there's nothing at all you could have done about that.
If one of the missiles spawns on top of you on Pictochat, you were in a bad position to begin with, and your opponent PROBABLY forced you there. Now, on the other hand, if the locations of the cart, flower, spikes, arrows, and missiles were randomly placed on the stage, and there was nothing you could do to preemptively avoid them, I would be more inclined to agree.
Pictochat's general philosophy revolves around controlling the left side of the stage, or you take large risks. This is a factor easily controlled by the players. In addition to that, the drawing time for each transformation gives a reasonable amount of time to react to most transitions.
That said, if you happen to land a Warlock punch at 30% on a light character, and you get the diagonal line spawn in and save them by bouncing them to the ground, that's really unfortunate for you, but could easily be considered a wise counterpick by your opponent, or a poor counterpick by you.
Pictochat is a calculated risk. Either take it or don't, but you'd better be prepared that your opponent might take said risk.