I've encountered toml a couple of times but I wouldn't call it wide spread. It's alright for small configuration files. However, if you keep things simple, json and yaml are also not so bad and even properties files or good old ini files will work. Doing e.g. cloudformation stuff in toml is not a thing though and it supports both yaml and json. If your data is simple, use a simple format. I've always liked properties files with simple name value pairs separated by =. Still very common in the Java world though yaml has replaced a lot of that.

BTW. I've handled all of those formats using jackson on Java & Kotlin. It has a flexible parser framework originally intended for json. But it has lots of plugins for different tree like configuration files. Look for jackson-dataformat-yaml and jackson-dataformat-toml on github. There are loads more formats that you can support with jackson. Nice if you need to translate from one to the other or need to support multiple formats.

IMHO Json with some tweaks would be really nice. E.g. just supporting comments and multi line strings would make it a lot nicer. A lot of json becomes unreadable due to the need to escape strings. I've come across Hocon a couple of times (jackson-dataformat-hocon) and it's a strict superset of json, which means that if you accept hocon as input, you implicitly also accept json.