no man ... anything more than low diff is head canon
the only reason you are trying to make a case for Marco high diff Big Mom is to claim the same about King
in case Zoro took any major damage, then you can claim
well Zoro is still a +high diff fight for Big Mom
while in reality Marco never damaged a single top tier
Big Mom has also never damaged a single top tier for the record.
One thing to note is that Marco never used a named attack on Big Mom but they had an intense clash judging by Big Moms facial expression and shockwave while Marco was smiling. Then Marco injured her homie but Big Mom managed to do no damage to Marco and then admitted she isn't capable of easily disposing Marco and literally ran way.
Its a clear portrayal than Big Mom can not low diff Marco you can argue mid diff but even that is kinda pushing it.
I am willing to acknowledge Big Mom can mid/high diff Marco and King and can very high diff Zoro.
Luffy can probably give her extreme diff