We have no idea what you did wrong because you did not tell us how you made this mp4.
When you check the composition settings does the word custom appear? If it does you should be using a standard preset for high definition video. 1920 x 1080 at 29.97 is the standard most folks should be using. Using custom sizes requires extensive knowledge and understanding of compression, formats, media players and custom frame size and frame rates should not be used by anyone who is not an expert.
When you chose the font and font size for your project did you consider the effect of scaling in media players or consider that very thin type does not often work well on video? To make fine lines work well they need to be very accurately positioned as close as you possibly can to the pixel grid. If you have no idea what that means then it's time to do a little research.
When you picked the color for your background and your text did you consider the difficulties involved when compressing solid colors against white, especially if the colors are moving?
When you rendered did your render settings or the settings in the Adobe Media Encoder say custom? If they did how did you change the default settings. The presets are there to give you a break if you do not understand compression.
Last question, how did you check your composition before rendering? It is really important that you understand how the composition panel works.