a) first, I think that you'd better start by a good reading of data :
float Temp = dht.readTemperature();
// because direct float-> byte conversion as you do looses precision
float RH = dht.readHumidity();
b) Then you need to convert your floats into a char string to display them :
double _temp = +15.5; // or float if you prefer
char s_temp[ 8 ]; // arduino's implementation of sprintf can't handle floats and doubles
dtostrf( _temp, 4, 2, s_temp ); // so we convert double to a char string with dtostrf()
c) At this point you can use display.print(char *) to display your data
d) Last, most of the time, when a char garbage is displayed AFTER a string value, it is because the string is not correctly ended by a trailing '0' char, and that can occur if the lenght of the string we allocated in RAM is too short and we write after its end.
so you can try
memset(s_temp, 0, enought_space);
to be sure.
if it can help you,
Which arduino are you using? You are likely running out of dynamic memory (RAM), the adafruit display library does not allocate the display buffer at compile time so the compiler will not warn you of low memory. Using a display and SD library at the same time on an UNO or other arduino with only 2K of ram frequently runs into memory problems.
Have a look at the U8g2 library (can be installed from the library manager), it has options for using a page buffer, which is much smaller than the full display buffer used by Adafruit's library, as well as a text-only display that does not need a buffer.
Although the image is not clear, I think you are driving a 128x64 set as a 128x32. Hence you will get the striped text look and corruption for say, graphics.
When properly driving it as a 128x64, the text will be clear and crisp
I have used that library and this initiator works:
Adafruit_SSD1306 display(128, 64, &Wire, OLED_RESET);
You can see that you can set the resolution there.
Some other libraries you have to make that change in the library file itself before compiling.
Our website has a sample usage of this (excellent) library (in the download section)