The way you guys all feel about this arc's plot is the SAME way that our MC himself feels about it:
bored, annoyed, and exasperated at how cliche it all is.
@Arcverson
You're right in that the author's intent probably isn't a revenge-gasm like Shield Hero.
We readers as well as MC himself know that MC WILL win. Heck, the manga title straight up tells us that MC will win ("the Strongest"), and the MC even compares this mook with the demon king (monologuing, strong physical and magic, etc.).
For all intents and purposes,
this self-proclaimed god mook is an inferior demon lord.
Imagine that you got turned into an isekai hero. You go off on a life-changing, emotional, and challenging adventure to defeat some demon king and find a way home, forging all sorts of relationships and enduring many trials along the way.
After many epic battles, you finally fell the demon king and end your quest, returning home.
At home, you attempt to return to normalcy as best as a endgame former hero can, hiding your powers and experience to the best of your ability and maybe making a couple new friends.
Then, some God-complex, arrogant, overwhelmingly-cliche inferior-clone-of-the-demon-king shows up and beats up your new friend(s).
Unlike with the demon king, you've got practically no emotional attachment to this villain character beyond him beating up your friend(s).
He's the most punchable thing you've met in a while, and his obnoxious, cliche monologuing is only making things worse...
He's constantly bragging about how pathetic and weak and inferior you are, and you are well aware that you could annihilate this mook like Saitama does with practically every enemy he fights.
Which is more satisfying for you: beating his pathetic "god" ass into submission/death with only your fists, or oneshotting him before he can even react?