Q:

Tooltip onTap rather than onLongPress possible? 

there's no named parameters to configure a single tap to trigger Tooltip,
my feeling about the default longPress interaction is that users cannot find this deep-buried function.  
I tried to find some hint in tooltip source code but failed.  
 Tooltip(
   message: 'this is something',
   child: SizedBox(...),
 )

A:

The easiest way to get a functionality you need is to clone the original Tooltip widget (call it e.g. TooltipCustom) and change inner GestureDetector behavior. 
Particularly replace onLongPress to onTap:
class TooltipCustom extends StatefulWidget {
  /// Creates a tooltip. 
  ...
  @override
  Widget build(BuildContext context) {
    ...
    Widget result = GestureDetector(
      behavior: HitTestBehavior.opaque,
      onTap: _handleLongPress,
      excludeFromSemantics: true,
      child: Semantics(
        label: excludeFromSemantics ?  null : widget.message,
        child: widget.child,
      ),
    );
    ...
    return result;
  }
}

P.S. It's possible to lose a tooltip hiding feature.  Take a look at _handlePointerEvent(PointerEvent event) handler function and realize a proper call of _hideTooltip() method.