I think a tiny house is just a very visually appealing travel trailer. They function much the same once you're inside.
In our area, any place where an individual could have a little place of their own, with out roommates, commands a premium as rental. Add a view and a pet, and I could rent as many of these as I had available. We live in an area where there's not a lot of affordable housing, and the demand is high. The deterrent to me is the entry price. Like single wide "park" trailers, they are not cheap to buy. If you can find a used one, that's the way to go. They can be very cute, with little lofts, and big windows. I've been hunting for years for a nice single wide to relocate to our area, but the resale ones generally don't have updated fire prevention systems, now required in my high risk for fire area. Utilities and septic or sewer can be burdensome. In my area, you're either going to go legal (prohibitively expensive for a $750 a month return) or slip it in (risk of complaining neighbors or code enforcement bringing the whole thing to a halt). We never found anything compelling enough to move on. By the time you're done making it legal, you might as well have something bigger that you can rent for more.
One of my best returns on investment was a used double wide manufactured home. It was empty and the owners were in a hurry to get out of the space rent at the mobile home park. We paid less than 20K for a smallish 3 bedroom, moved it, set it up on a permanent foundation, installed the fire prevention and hooked it to the utilities. The utility part was pricey, but had to be done anyway to develop the lot. That was maybe $50-65K total, but would be more now with the fees. That was 20 years ago and I've been making 20K+ a year on the rent ever since. Yes, about $400,000 on it! I don't think it's been vacant for more than a couple of weeks total.