How do you make pixelated iPhone videos look better?

I have to make an AP Chemistry video in iMovie for a group project, and one of the kids in my group took the video on his iPhone (4 or 4S), but it looks extremely pixelated! It didn't look like that when I saw it on his phone at school.

He texted it to my email. Is that the problem? Is it a problem with the way my computer is displaying it or something?

Is there any way to fix it? I'm not sure what to do.

Some research has revealed that it may require "deblocking," but I don't know what software to use that is compatible with my Mac.