NOTES:
- I'm posting this as per the request made here: https://github.com/mattermost/platform/issues/2549
- I am not capable of implementing this myself

Mattermost highlights user's own messages with a gray background (color depends on fg text color). It is entirely up to user's personal taste and preference whether these highlights add anything to the experience: I personally find them distracting and feel they disrupt the flow of the conversation. Also, visually, they're unappealing.

So, however you feel about these highlights personally, you should at least agree that they're a prominent visual feature. As such, users should be able to turn them off from the customization menu.