I suppose depends on what you consider long term... like all summer long, sure it could work, just like a kiddie pool. Multiple years long, probably not so much.

I'm thinking yeah like a small pond or kiddie pool in the backyard, that could be drained in the winter and filled in the summer.

But wont the water get like super gross really fast? with algae and germs?

If you don't keep it clean it will get gross, just like really big permanent pools. Keep it clean, use clean water if needed, or pool chemicals, and it should last for a summer just like a kiddie pool would. Using a pump of some sort to keep water circulating like with bigger pools would help too.