If I try to create a trait that affects another character (example indecisive) to make his rolls more difficult, should it be an opposed roll since it affects him directly?
Interesting. I would say yes.
Although ideally you’re create traits to help yourself as technically you’re talking about giving another character a complication (which is fine and still a trait).
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I was about to say that it would still be a Difficulty 2 action. 
On the basis that the rulebook doesn’t have anything about contested rolls on creating a trait.
I was going to add however, that when you create a new trait (either through D2 action or momentum) it must relate to the action you’ve just attempted, and it must be something that could reasonably result from that action. (Pg 152)
LOL, it is certainly a GM call thing, and very much depends on the situation and what the PC is trying to do with it.
You can argue a full intrigue might be used to give a complication to another character just to weaken them for the next intrigue with higher stakes - like poisoning Bond before the big poker game. 