There were legitimate reasons from a design standpoint. It's badly balanced, the spacing is inconsistent...and it was everywhere.
Funny enough, I suspect what makes it a badly designed font might be why some people with dyslexia have an easier time reading with it. The badly balanced, poor spacing, probably made the letters in the font more distinguishable from one another.
If you (or anyone else that's interested) have the time, I think this article, "Why You Hate Comic Sans," goes over all of it pretty well.
According to their Github issue tracker, they're aware and are working to fix it (based on the pull request)!
The lemmy software is still a bit rough. It can only get better though, we all just need a little patience (or to contribute if we're able) π
As far as I know, I don't believe there's an FAQ that explains these issues. I could be wrong though!