We don't have any experience with RStudio running on the Amazon Linux distro -- I do know that we rely on an installation of the pangocairo library to render text, and it may be that this library is either not installed or not configured properly on the EC2 instance. On Debian and RedHat systems we install this (respectively) using:
sudo apt-get install libpango1.0-0 sudo yum install pango
Note that R also has an (optional) dependency on pango cairo that is nevertheless highly recommended. See:
Hope this helps!