Your issue may relate to this threadhttps://forums.mirc.com/ubbthreads.php/topics/262869/irc-ansi-art
Since you didn't provide a way to reproduce your problem, I can't test to see if a different font like the one mentioned in that thread would evade the problem. If you're using something that contains characters that get garbled by the forum, you can put the entire multi-line string into the clipboard and then paste into your post the output from:
//var %a $encode($cb(-1),m) | echo 3 -a %a | clipboard %a | echo 4 -a $decode(%a,m)
The green is what you'd paste, and the red confirms that you don't have something unexpected in your clipboard such as the above command itself
Unfortunately this kind of ASCII Art is "in the eye of the beholder". There's other forum users much more familiar with this activity than I am.
If mIRC were fixed so that there wasn't a guaranteed pixel gap between lines, that might affect the readability of normal text being crowded together, unless these kind of codepoints were able to be full height while letting text letters avoid touching the text on the row above it.
And even then, that would only help the viewing by those who had the same client+version+font settings as you have. i.e. someone whose channel font is a proportional font like Arial won't see this kind of display well if it uses different characters who have different widths in a proportional font.