My daughter and I will be going to Ecuador in September this year, and she would like to know if wearing general makeup (foundation, mascara, and the like) will be a problem for her due to the heat/humidity/etc?
It highly depends on how much you are used to the temperature and humidity, and how you will get around.
If you are most of the time in a airconditioned truck, no problem. If you walk around outside, it will be the same as if someone takes a water spray and sprays it over your face, every 10-15 minutes or so, depending on your fitness level.
If you are not used to 80%+ humidity, be ready for a completely new experience. You will be sweating - a lot. And there won't be a nice breeze to cool you. And the sweat won't dry off because the air is so saturated with water that something, once wet, stays wet.
So once you start wiping your face to get rid of the water/sweat, your makeup will be gone, too. So better leave the makeup at home. The animals in the rainforest won't wear any either, so everyone is on the same terms there.
Not every place in Ecuador is that humid though. The high mountains are drier, so it might be worth it to check the local temperature and humidity. This website has quite nice charts regarding this issue. But also watch the rain levels. Humitidy does not really matter to you when you are in a tropical rain. If you are not familiar with those: It does not make sense to bring an umbrella. Even if you bring special water-resistant clothing, you will sweat in it and be pretty much wet, but will stay relatively try compared to be exposed to tropical rainfall. The rainfall is more comparable to jumping into a pool. September should not be that bad in most of the places though.
Added info: If you plan to do any strenuous exercise, make sure to carry loads of water - even if you consider yourself fit. I live in a tropical climate, and we repeatedly experience quite fit tourists needing to be rescued by a helicopter of the mountains because they did not expect to dehydrate that quickly.