Actually I think it's both.

I've observed that many grossly over-bored engines (and the ST90 fits into this class) have quite shallow transfer ports and that causes a marked intolerance to anything but a perfect air/fuel ratio.

Some engines (like the OS55AX) get away with this because their carbs are very, very good at maintaining an accurate air/fuel ratio at all throttle settings.

However, the ST carb, combined with the narrow transfer ports means that the ST90 is very difficult (and in some cases impossible) to tune to get a crisp mid-range transition.

Some folks get close by carefully selecting props, plugs and fuel (all of which can go some way towards mitigating the design problems) but the limitations still apply.

Others have found that other carbs are "just" good enough to provide a better (more constant) A/F ratio at intermediate throttle settings than the ST one does.

So, a different carb can go a long way towards solving the problems but won't necessarily cure it.