I thought he was mainly a philosopher since his theories are mostly philosophical (or at best quasi scientific). Since he is rarely cited by philosophers it makes me wonder if he is considered a serious philosopher by researching philsophers?
No, he is not.
A good indicator for that is the fact that C.G. Jung does not have an entry in Stanford Encyclopedia of Philosophy. A man of his acamedic weight in psychology would certainly have such an entry if his work was considered influential in philosophical circles as well.
Freud does not make the cut, either. However, the much less broadly known Wilhelm Maximilian Wundt, for instance, is in. Not only is he considered the father of experimental psychology but he is also accepted as a philosopher of mind and, hence, as a philosopher in his own right.
In my opinion, in the absence of a deep empirical investigation into how many philosophical papers refer to Jung, this settles the case quite well.