Just speculation. Running a steam railroad locomotive you would fuel (fire) the boiler at a rate close to the speed or tractive effort required at any given time. Running the boiler hotter would not only mean venting excess steam (a waste of power), but you would also likely have inefficient combustion -excess smoke being a tell tail sign.
The carrier is likely traveling at far less than maximum speed, but with potential enemy ships with just thousands of yards away they would need to ability to meanuver quickly -not waiting to build a full “head of steam”.
So, you might assume that the boilers at running at full capacity, grossly inefficient given the speed required, but excess capacity is lending the ability to go to full speed quickly -without lag time to build more steam. Again, running in that inefficient state it is expelling unburnt gases (inefficient combustion) and likely venting excess steam.
This may well be why the smoke plume looks rediculous.
No modern steam driven warship should smoke black like that. The reason it is smoking black is poor training and /or old boilers. It could be bad fuel but I doubt it.