In my opinion its good to do demo jump maps, you get to know how the sticky actually works

Play more demoman.

play demo in pubs, badwater bestwater

practice sticky sniping.
just charge it longer, in general that'll work better for you

Pubbing is actually pretty useful because they're almost pure dm. Slap on some wee booties or bootleggers and try to aim your stickies instead of blindly throwing them and detting them midair. You also should try to delay detonations at times so opponents get hit by multiple blasts at once instead of the same individual air det spam. Demo jump maps are also amazing for sticky control, and they help you learn sticky trajectory and detonation timing. And also pretty much always charge your stickies so they can move as fast as possible when you shoot them, unless you're going for a chained simultaneous detonation.

On a sidenote, why are there so many threads about vague aim improvement cropping up all of a sudden?

Maybe that's because ppl care more about skills now...

I heard that ppl say that no piping is no demo at all, when should i actually pipe at all? like facing a scout?

There's a bit bigger chance to survive a 1v1 vs Scout if you use pipies I think, I'm not a Demo main, so I dunno

You should use your pipes to spam through a choke, spam over a long distance, or whenever someone's really up close.

one time i put a sticky at my feet and a scout ran into it and it one shot him

public & lobbies with stickies only :)

Use both weapons.

Control where he can move with the drop sticky and switch to pills or cannon balls. That's your best bet with Crackhead 2 jump man unless he's simply too close.

The first video in this blog post from warhuryeah's demoman blog is really useful for what post #6 touched on

http://warhuryeah.com/2014/03/04/decimating-the-opponents/

That whole post is good, and I'm sure his Aim Theory and map-specific posts might help too.
Good luck!