UN Treaties have no legal power in any country. Each administration that signs the treaty must implement it in laws in their own country. So, there is no way that this move represents UN control over American citizens Constitutional rights.

If the US congress chooses to ratify the treaty, then it will mean restrictions in the trade of weapons to other countries, and that in my opinion would be a very good thing.

Iran has been active in the discussions leading to this treaty proposal. So what? This is normal. Iran maintains a very high profile in all UN Organisation meetings these days. That is what you do if you are paranoid about the world hating you, or rather that should be : if you are correct about the world hating you. Other UN member nations have become quite adept at handling provocative interventions from Iran, so there is no serious danger of it having an adverse impact on this important treaty.

The only Americans affected will be criminal arms dealers, and they have no Constitutional protection.