How to solve "A RenderFlex overflowed by 143 pixels on the right." error in text?

Just Wrap you - Card with -Flexible Widget.

Row(
    children: <Widget>[
      Flexible(
        child: Card(
          child: Column(
            crossAxisAlignment: CrossAxisAlignment.start,
            children: <Widget>[
              Text("Item name mmmasdaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaammmmmm"),
              SizedBox(height: 15.0,),
              Text(
                "Discount mmmmmmmm",
              ),
              SizedBox(height: 5.0,),
              Text(
                "Price ,mmmmmmmmmdgfdgfdgfdgfdgfdgfdgfdgfdgfdgfdgfdgfdgfdgfdgfdgfdgfdgfdgfdgfdgfdgfdgfdgfdgfdgfdgfdgfdgfdgfdgfdgfdgfdgfdgfdgfdgfmmmmmmmmm",
              )
            ],
          ),
        ),
      ),
    ],
  ),

You can use Wrap instead of Column.

For ex..

new Wrap(
   spacing: 5.0, 
   runSpacing: 5.0, 
   direction: Axis.vertical, // main axis (rows or columns)
   children: <Widget>[
       TitleText(text: "Item name mmmmmmmmm $index"),
       Body1Text(text: "Discount mmmmmmmm",color: Colors.red,),
       SubHeadText(text: "Price ,mmmmmmmmmmmmmmmmmm",color: Colors.red,)
   ],
 )