Hello!
Well, right now, the temperature in Shimla is about 22°C with about 83% of humidity. The weather is mildly pleasant, not overly cold nor hot. Generally, from July to September, Shimla is cool and humid. The difference is that, humidity in colder places makes us feel colder, not hotter.
For example, in Delhi, when there's a humidity of say, 90% and the temperature is about 45°C, then you can say that the humidity makes it hotter. Why? Because humidity makes it harder for the sweat to evaporate and sweating is a natural cooling mechanism of the body. So when it is a hot day with a high humidity, there's lots of water vapour in the air and that makes it harder for our sweats to evaporate. That is what is giving us that feeling that the humidity makes us feel hotter.
Now, let's go to Shimla, where temperatures are low but humidity is high. For example, with a temperature of 22°C and 83% humidity, why doesn't it feel hot? Well, because high humidity in cold weather increases the conduction of heat from our body. For example, the humidity in the air can cause some dampness in our skin or clothes and this conducts the heat away from our bodies. Another reason is that Shimla has particularly cloudy days which shields the solar heat away. We don't particularly sweat on a cold climate even if it's humid so the high humidity doesn't limit the evaporation process as well.
So I still think Shimla is worth visiting. It's one of the best places to escape the summer heat in lower altitude areas of the country. As I've explained, although it's humid, the weather and climate in Shimla is still cold and pleasant especially just before the peak of winter season. Winter season has particularly colder temperatures even as low as 0°C.
I hope this helps!