Is it ok to power a Wemos D1 R2 placed outside with a smartphone charger?
It might be okay, but it's better to play it safe. I would recommend putting the electronics (wemos/charger/extension cord) in a ziplock bag or plastic container designed to keep food in. You can cut holes for the sensors and power and then seal them in place. Have a look at this post. To answer your question. If water were to seep into the charger, it would have the potential to short the mains power in the charger. It could at the least ruin the charger. It might trip a breaker and or cause damage. Depending on the quality of the charger, water inside could expose the wemos to mains. 
The charger may consume a small amount of extra power, but it once again depends on the quality. Apple chargers, for instance, do not. It won't be extreme, and is unlikely to go noticed, regardless of the charger.