Everyone can be humble for a day or resilient for a single task, but one can not so simply create bonds if there aren't any. Not without a Primarch noticing that something screwy is going on at least.
Humility doesn't matter.
Of course even Big E will work a bit, that's a small price for securing a Primarch (and a planet too).
It absolutely matters,
in this context
Because the question Mortarion asked is specifically-
"You call yourself Emperor," you say at last, "ruler of men, forger of monsters. How am I to know you are not an Overlord, kin to those I slaughtered?"
-he's asking the Emperor to show him he's not like the Overlords of Barbarus, who he previously noted with grim amusement,
would be furious to see him working in the field. They were haughty, proud, and unbending men who used their psychic powers to dominate others.
As for Camaraderie... again, this is an area where Big E's natural talents HURT his case. This Mortarium clearly has much more honed psychic senses, he thought the Emperor was a Daemon based on the power he radiates without trying. Add on to the fact that at this point in the Crusade... the HYPE is real.
Unless Maugan has done some tweaking... It is highly unlikely anybody on the Ships above will be able to give Mortarion an actual objective opinion. ESPECIALLY not the Custodes. Hell, I'm reminded of the poor Remembrancer who dared to voice his doubt in the Great Crusade, and was promptly beaten near to death for it.
That level of fanaticism, plus the power Big E is pumping out, does not give a good impression when you are already suspicious.
-Edit-
Mortarion: "How do I know you aren't a monster?"
Emperor: "Oh that's EASY! Just ask the
heavily altered and indoctrinated children of the warlords I killed! They'll vouch for me!"
Custodes: "ALL HAIL THE EMPEROR!!!"