Sorry I had to say that, I know how depressing it can sound. Note that we're in the context of recommending examples to someone wanting to learn about react, so there's an expectation about finding the best code around.
The general critic I would make about Mattermost's React source code is that it does not take advantage of react composable nature, and instead just drop all the code in huge render methods, which are fairly difficult to read, and probably to maintain.
Here is an example: https://github.com/mattermost/platform/blob/master/webapp/co...
The render method takes 170 lines, putting tons of "stuff" in. This file could easily have been split into 15 to 20 components to make it easier to reason about.
Same here: https://github.com/mattermost/platform/blob/bb69e98631b25419...
Almost 200 lines in the same function, containing rendering, looping, conditional execution, more rendering, that is, about everything. This should be split into more components and more functions. Most of files can be taken as such examples.
I think that's actually a problem for Mattermost, because when you run someone else code on your server, you take the responsibility for security breaches. I know how horrible this sounds, it's super cool to be able to run a slack-like app for free, self hosted. The thing is, reading the code, I'm not confident the tools used has been properly understood yet, which in turn makes me reluctant to execute it on my servers.
I wish you the best, though, you're on something with Mattermost!
EDIT: please also note this is not just Mattermost. I usually review part of code of apps I want to put on premise, and decide whether to keep them or not based of my level of confidence in it.