explain the sentence" That's a bit mean"

I listen to the sentence from a conversation in Englishtown. I don't know what it means. I guessed  the meaning, but I don't know whether it is correct or not. Could you please explain it in more detail ?

Below is the short extract from the conversation

A: "...I'd like to start my own business making ice cream that never smelts"

H: "Well I wouldn't get too over the moom about that yet> I mean you no rocket scientist"

A: That's a bit mean"

Hello thuhuong,

Well, the first person wants to create a new kind of ice cream that would never melt; the other one in fact tells him not to dream too much ("don't get too over the moon"). So logically the first person is a bit offended, and "That's a bit mean!" simply means "what you are saying is not very nice", only stronger. "Mean" means nasty, bad, unsympathetic.

Hope I could help. 

Yun 

Dear  Yunie 75,

First of all I'd like to thank you very much for your edequate explanation. which is very helpful and satisfactory to me. 

I'm Vietnamese. I live in Danang, a coastal city in Central VietNam. I'd like to make friend with you.Can you write to me?

Thank you once  again

Best regards

Thu Huong

PS. Could you please correct my mistakes if I have done

Dear Thu Huong,I'm glad I could help :)  Yes, I would write to you, but I couldn't find your mail on the "Find friends" section, so I hope we can meet in the chatroom sometime…
All the best,
Yun