This name would never be given to any child, and it's only known as a mythological character. The name would be far too pompous for a person, and I doubt it would even be legal to name your daughter this. Moreover, the -tar ending is the Finnish equivalent of the -ess ending in words like duchess and lioness and such words. So, you might say that this name means ''air-ess'', and anyone named this would be laughed at. Or maybe admired, depending on how cool they are. I wouldn't recommend anyone to try.
There is absolutely nothing illegal about naming a daughter Ilmatar. The name is in the name lists that are printed into official calendars. According to the statistics of Finland's Population Register Centre, the name has been given to over 60 children after the year 2000. Personally I wouldn't be surprised to hear this as someone's middle name. As a first name not very practical to my ears, too long. I also think that to a person who knows any Finnish it's bound to sound nothing but feminine thanks to the "-tar".