There are no rules specific to EL&U regarding usernames.
Over on main Meta I could find 3 possible prohibitions:
The basic guidelines seem to be:
- If you find a username offensive, flag it and a mod will deal with it at their discretion.
- Weird characters in usernames would be actively prohibited if developer time allowed, but there's no need to flag usernames with such characters unless you find it offensive.
- Advertising in usernames is acceptable. A username is essentially treated as part of the profile for this content.
So with regard to "Tiago supports GoFundMonica", that name seems to fall under "it's OK on the profile, so OK in the username."
There was a period where actual links to Monica's GoFundMe were deleted from profiles, but I'm not sure of SE's stance on simply mentioning it. Until they say one way or another I will assume it's fine.
As ColleenV commented, there is an SE meta thread on this subject.