Not, it isn't.
In fact, as far as I know, Monica
is not illegal in any place (I checked the information with all countries where I know that there are some naming regulations, because I'm particularly interested in this name).
Sometimes, some people's comments to names are not reliable. AndrewJKD (the author of the comment about Monica
as illegal name) is only 13 years old and perhaps his sources of information about names laws are not as good as they should be.