Honestly, I'm hoping that next page the long time players are collectively a little shaken by Miss Valentine's ability. After all, it's technically Weaponized Fall Damage.

Luke wouldn’t care and considering certain events in the future I don’t think fall damage is that big a deal for this system

Sides, bigger thing is that she can turn anything into "light" weaponry. And "heavy" weaponry as well.

Hell, if you wanted to be cruel, she could weigh down a person's clothes/armor so much that they couldn't move/were crushed by their own clothes.

She never actually shows the ability to use her weight change on things other than herself, except possisbly her clothes and most fruit powers do that-ie logias not stripping all the time and people like buggy not running around in rags. Maybe she could but she never does in the manga when we see her. I have seen it in one or two fanfics though

In all honesty, that sounds like what her Devil Fruit's "Awoken" ability would be, since the one we've seen took a personal ability and allowed it to be projected onto other objects.

Possibly, but there's so much we don't know about Awakening's mechanics that it's hard to really say.

I've always been fond of the theory that Machvise (Dressrosa)'s Ton Ton Fruit allowed him to manipulate the weight of objects, since he always carried around that bigassed bronze shield when he was using and otherwise it's mechanically just "Kilo Kilo fruit but way better."

Oda in an SBS actually does compare the Kilo Kilo and the Ton Ton fruits as "magnitudes" the same way that Fire and Magma are compared. There are canonically some fruits that are just "this but more". The sea is a cruel place.

So, is common for the GM's to just overkill some NPC and don't give the player the chance to try to even help?

I wouldn't say common, but I would assume in this case it would be to drive home how terribly dangerous their opponents are in order to push the player towards a desired plot point.
It doesn't tend to work well, since many players assume anything on the board is killable (and if they have stats, they are), and so will blitz the bad guys no matter what, trusting their build to overcome the challenge.
And then get cranky when the big bad that was only there to monologue and leave swats them down because he's 7 CR above their pay grade

Kinda depends on the game, too, though.

Players of OSRPG stuff usually learn *not* to go into the game assuming that they're meant to be able to beat everything that appears, and learning to avoid fights entirely or run the hell away when something too big shows up

Depends on the GM.
I try to only ever do it when it's dramatically appropriate.
Of course, I'll also have a random zombie just spend a few rounds nomming a downed PC so...

If you take a look at the GM's text boxes, you'll see he's been rolling dice. I'm assuming he's getting really good rolls for 5 & Valentine but bad rolls for Monday. Considering that last one before the big ka-boom says "<Rolls> Er... hmm..." I don't think he planned for it to be this bad.