I’d like to learn to hop with my uni but I don’t own a mountain unicycle but just a normal ~100$ uni. Is it ok (i.e. no broken frames, damaged wheel, etc.) for the uni if I practice hopping or do I need a Muni for that?
You wouldnt want to hop high on it or put to much stress on it. You would likely bend your cranks or bend the rim in half if you’re not careful.
Hopping on it should be no problem at all. On my unicycle a buckled the rim and bent the cranks but I got up to 3 and a half feet drops on that. I’m light (50kg or so) but still just hopping really shouldnt do it any damage.
But a Muni is mainly for mountains (Mountain Unicycle) if you just wanted one for hopping around on I expect you’d be looking for a trials unicycle. Hopping on Munis is fine, most people just use trials unicycles isntead for trials 
I wouldnt worry too much about it, if you’re learning to hop. Hopping up things should be fine, i learnt on a stock standard 20" with no handle, and i didnt do a bit of damage till a twisted crank at a 5ft drop, anything below that was fine, so you never really know your luck.
Depending on what you want to progress into, thats the type of uni you should get. i bought a 24" muni, which is also a nice uni for doing trials on, as well as the off road and on road. but, i am getting a trials uni soon, as street riding on a muni tyres wears them out real fast
Hopping
When you start hopping, start saving up… I bent the hub on my ~$100 uni from hopping too much… well also muniing with it 
Learn to hop, and when you start breaking things upgrade… That’s what I did… It can take it for a while, but eventually it’ll break. I saw a kid who prolly weighed maybe 130lbs bend his rim hopping off a picnic table bench last week, and even then he was able to keep riding for the rest of the day… Though you could see the rim starting to taco when he hopped it after the drop
I managed to crack and eventually total my uni frame as well as taco my wheel. I fixed both problems temporarily but have permanent creases in my rim. Get a sturdy uni asap.
I have been hopping on a new cheap 24" norco unicycle for 1 month and already my unicycle’s hub is twisting. Also my crank arm falls off every 10 minutes of riding, could it be concected to the twisted hub? I haven’t done any drops with the unicycle just little hops and riding off curbs. Does a cheap cotterless hub break or generally just keep bending?
I take my chrome uni to the skatepark all the time. its only a 20" 100$ yeah, but it holds up well.
i do hopping going up/down stair. riding down stairs and drops of up to 2ft on mine. i wanna wreck it so as i can get a new 1 but its not working… 
keep in mind i only weigh just under 40kg so if youre much heavier than that you might wanna think twice about doing some of that stuff on it. hope this helps.
I am 75kg and am using an adult learner unicycle from Butterfingers. I haven’t really bothered with drops yet, and other than having to replace a faulty hub early on, it is still holding up now that I am hopping up and down steps, jumping over logs etc…
I had to change my seat, once I started hopping I practically ripped my original seat off in a couple of weeks, and upgraded to a KH seat, much better.
I also changed my tyre, upgraded to this: Snafu Job Rim HP . I have found it ideal for learning, the high pressure really helped in the early days, gives you little bit of extra rolling momentum. Now that I am hopping, I have dropped the pressure to about 60psi to get a little extra bounce. The grip is perfect for the streets, fairly hardwearing (which I certainly needed initially), and gets me by when off road as well.
I will be looking to upgrade to an Onza or KH 24 as soon as funds allow though. 
thats a nice tyre! for learning, i would recommend the Maxxis Hookworm. it does a nice high psi (110 if i remember right) for freestyle tricks, and its big enough to let down quite a lot for hopping