Mastodon isn't a Twitter clone but it was made to fill that exact niche. Short posts with mentions, hashtags, and URLs. Such details give the feel of a platform. The velocity, the atmosphere. On a personal level that's just what I like. And I think it would be reasonable to assume many of those who signed up do, too.
A social network is also a kind of common playground. Individuals have the capacity to spoil it for others. For example, when there was a bug in Mastodon that allowed you to make links spin, many made use of it for fun, which spoiled the experience for people with motion sickness. The more vectors there are for how information is transmitted, the more pronounced this is.
Right now, there is not a whole lot of ways that individual posts can escape their boundaries to affect the whole experience. They can use lots of line breaks or a wall of text, but it's limited by a maximum height. They can use caps, but they don't stand out that much. Custom emoji are one of the weaker spots when people use them as fonts, and that's annoying.
But adding more elements to the mix? More control over visuals? People WILL make it look ugly, that's a guarantee.
There's another angle to this. The fediverse is large, and its participants who are not Mastodon have different ideas and design philosophies. Some of them are publishing all sorts of lists and quotes and strike-through items. And when those posts show up on Mastodon, those visuals are lost.
That is not ideal, when it happens! But interoperability between systems with different goals and capabilities can't ever be full, it's a spectrum.
Unless you seek to emulate every system you interoperate with, you have to draw lines. And drawing the line at what Mastodon was designed to do, kind of makes sense to me. The text is the most important part, and we've got that.
Fosstodon is an English speaking Mastodon instance that is open to anyone who is interested in technology; particularly free & open source software.