"It's too hot today" is common as a statement. "Don't tell me it is 30 degrees C today" is okay. Among native speakers one would usually contract "it is" to "it's" instead. So... "Don't tell me it's 30 degrees C today." If you are among friends and/or people whom you know then you have options: "Seriously!? It's 30 degrees C today?" / "Are you kidding!? It's 30 degrees C today?" / "What!?" It's 30 degrees C today?"

When we talk about outside temperature in a conversational tone we wouldn't usually be so specific. It's too hot today! Is it really 30 degrees outside? This would be a more natural sounding expression. Don't tell me..... is more appropriate in situations that have to do with a person, although not always. It's 30 degrees outside, don't tell me you're going to wear those jeans! I just washed the car, don't tell me that now it's going to start raining!