Hi Somesh,
That doesn't look like a Brugmansia to me - I've only ever seen a Datura once but from what I know I think that's what you've got. Brugmansia's are
trees with downward growing flowers, all the ones that grow here are anyway.
I imagine there wouldn't be any issue with mulching it from a poison perspective. The chemical that makes it poisonous is Atropine, this slows the heart or stops it if you take too much. I can't say 100% but I doubt that this would affect other plants or pose any risk of getting absorbed by others. One thing to take in to mind (at least with Brugmansia) is that they are listed as a
hyperaccumulator , I've read that Brugmansia take up heavy metals from the soil - there aren't any details on wikipedia what exactly they take up. If you have contaminants in your soil the Brugmansia may have concentrated them and so mulching it could lead to you releasing them again.
Matt.