As an engineer who's worked on very large codebases over two decades, I've realized that this is so much easier said then done.
If people want to fork Mastodon, great. But they'll quickly realize that what they may think are straight-forward "improvements" will lead to them having to address bigger architectural issues.
Many design decisions that were made when building Mastodon may not be perfect, but they address a lot of very complex decentralization and federation issues.
There's no such thing as perfect software. What some may think is an improvement, others will think is a terrible choice. Each decision is a trade-off and will have downsides. We just have to decide which of them we're comfortable with living with.