The CGI was done in 4:3 for budget reasons, so CGI and composite shots are only available in 4:3 (which then had to be cropped for 16:9 later on), which is why the CGI looks terrible. At least in the earlier seasons, afaik later on rights were acquired by some network that wanted 16:9 and so from that point forward CGI was also in 16:9

https://www.modeemi.fi/~leopold/Babylon5/DVD/DVDTransfer.html has lots of info and examples