Yeah, he show proof when his credibility already reach a bottom and on the way to get fired.
Of course they would dismiss it as an Excuse from a loser
That's not the implication I got.
Here's the dialogue from that bit. Keep in mind, this is just
before he shows them the monster fang he got earlier that day...
"You always lie about killing monsters to skip work. You think your words can be trusted?"
"I wasn't skipping work! I really did kill monsters! I've shown you so much proof! I've also gotten rid of some monsters that appeared in the territory this morning."
This wording clearly implies that he's been taking proof of monster subjugations and turning them...
And that it's been an ongoing thing where they don't believe him even though he has that proof.
If you ask me, I'm betting it will come out in the next chapter or so that the butler was putting the idea into the feudal lord's head about the proof being fake. He might have been feeling that his position was being threatened by the MC.
Why?
Because the guy
probably was getting all of his work done, and
then going out to subjugate monsters on top of that. It's the same sort of short-sightedness you see in middle-level managers in a Japanese black company. They'll do everything they can to cover up their ineptitude while throwing their underlings under the bus - regardless of whether or not losing that underling is going to hurt the company.
EDIT: Come to think of it, the attitude that head butler was so close to "Black Company Middle-Manager" that it will not surprise me if he was the one who was actually slacking off, and was using the MC as a scapegoat.