There are some details from their interactions...
• Big Mom asked Perospero for help. A marine put sea stone cuffs on Marco and I don't remember the admiral that was present there (Kizaru) giving such an order or request it.
• The Big Mom/Marco interaction and the words used imply that it would have taken BM at least some time to deal with him and that Marco is annoyance to her. I don't remember this ever being implied by any admiral.
•Garp had a far better showing against Marco than Big Mom had and he is confirmed by panels to be weaker than the admirals at MF.
Now, I don't put Big Mom down for this and it wouldn't be wise to do so. And I don't think she is weaker than old Garp either. I also do think that Marco can be an annoyance for both her and the admirals and she is in the tier of strenght of the MF C3, just like the other emperors.
But if I wanted to debate in the yonko/admiral discussion, I don't think that bringing up Marco as a point against the admirals is a very good idea lol