One of the first sentences I learned years ago was “J’ai un mal du tête” for i have a headache. This was from a Parisienne. Was this less formal?

I think Rose Marie meant "J'ai un mal de tête".  I don't know if that would be considered less formal. This thread on WordReference suggests it's more complicated than that. 

https://forum.wordreference.com/threads/jai-un-mal-de-t%C3%AAte-jai-mal-%C3%A0-la-t%C3%AAte.732133/

One user says:

On dit "j'ai mal à la tête" ou "j'ai un (violent) mal de tête"

Dans la deuxième expression, il y a toujours un adjectif pour qualifier le mal de tête.

Another user says that there is often an adjective, but not always.

Just a slight correction: J'ai mal à la tête. This is perfectly ok to say, and it is certainly not slang.