The weird thing is, whenever I see a custom font (or at least when it’s noticeable) on a phone, it’s Comic Sans. It’s usually people who are not super tech savy, but still they went out of their way to customize their phone to show them all interface elements in Comic Sans.

As the expectation became "We should design our products so that the most technologically inept person can use it" customization in general got sidelined and a huge number of users are either unaware of what customization options still exist or lack the confidence or capability to take advantage of it. The result is that for most people's stuff, everything looks standardized and a small number of folks (the ones who have always loved customizing and personalizing everything) are the exception.

Perhaps those of us who like to customize were always doomed to be the exception rather than the rule since sticking with defaults is the path of least resistance and as long as something mostly works many users won't bother to change anything at all. It's also harder for the pro-customization crowd because eventually developers are going to consider something along the lines of: "The majority of our users use whatever defaults we impose on them, and having things more standardized can make it easier for support anyway, so why are we spending time/resources maintaining all these customization options that only a few people use and which could potentially make a clueless user confused and angry." which can lead to further restricting/limiting customization options.

It can get much worse. I have a friend who uses a "cute", handwritten-looking font on her phone. She is Chinese. When she sends me screenshots, I can barely read them. I have enough trouble with standard characters.

QQ took that one step further, and let you customize the font in which your messages appeared to other people. I don't think QQ is as popular as it was, though.