Is ping misuse reportable?

you can try it but you likely will get your report weight reduced from false reporting

No.

Harassment, so yes.

Haha, they need a "What the fuck noob?" ping so I don't have to use the missing one.

If they do it consistently (or spam it), I'll generally report them for harassment and/or spam with an explanation in the comments.

You CAN report it. But unlikely to be effective, Riot gives players the tools and yes some players use them to harass but its hard to actually PROVE that, beyond a doubt. I ping other lanes when I see someone go missing frmo mid, even as a top laner or elsewhere so I can make sure my team is aware of it. Some people might feel this is harassing them while they are in lane, but I am using the tool as intended.

Thats what causes the issue

yep ping spamming is reportable.