Android, Raspberry, Atmega, SQL communication
If I understand correctly:


You have a Pi hosting MySQL and a web server. 
A Mega writes temp/humidity data to the server. 
An android device reads data from the web server. 
The Pi and the Mega talk over a 433(MHz) radio?


Is it the write way to go?
What you have is a tried an tested design, a central server responsible for collating at serving the data.  My one concern is the 433 radios, I think you might be limiting the scalability of your design by using that.  What happens if you want to add a second Mega to report a different sensor?  You PI needs a second receiver and a second app to read, although the PI will handle 2, easily, there will be a point where it can't handle any more.  I'd suggest that you look at technology that allows multiple nodes on the same network, WiFi.

You could use WIFI by either getting adding a WiFi device to you Mega or ditching the expensive Mega and buying a $2 Wemos D1.

But that's just my opinion, and unfortunately that's the reason this question will get closed, because asking for people's opinions is taboo.