My mother was an American citizen who never lived in the U.S. : can I get a “green card”?

Your question is an interesting one. It touches on an area of immigration law that is rather complicated. Perhaps the better question is – did your American mother give, or “transmit”, American citizenship to you? If she did, then you are a U.S. citizen, too, and you have no need for a “green card”. If she did not, and assuming she is still alive, then she should be able to file an immigrant petition for you so you can indeed get a “green” card. I will need more information.
Let me try to explain. A person born in the U.S. is a U.S. citizen (except in a few rare situations). That’s simple enough. That is still the law, and unless the U.S. constitution is changed or amended, will continue to be the law. You might have heard or read about current efforts proposing a change in this law. But that’s all they are – proposals to change the law.
It’s when a person is born outside the U.S. where it becomes complicated. A variety of factors come into play. Were both parents US citizens? If not, who was the US citizen: the father or mother? Was the child legitimate? Did the parent or parents live in the US? When was the child born? The answer to this last question is really the most important because it tells us what version of the citizenship laws apply. Citizenship laws constantly changed over the years. 
For example, the residency requirements for a U.S. citizen to be able to transmit citizenship to his or her child born abroad differ for different periods. These required that the US citizen must have lived in the U.S. for a certain number of years at certain ages. Thus, a US citizen parent must have previously lived in the US for 10 years, 5 of which after 16 years of age, to be able to transmit citizenship to a legitimate child born outside the U.S. between January 13,1941 and December 24,1952. If these conditions are met, such a child is born a U.S. citizen. But the citizenship laws in place for that period also required that for the child to retain U.S. citizenship, he or she must have two years physical presence in the U.S. between the ages of 14 and 28. Otherwise, the child loses U.S. citizenship. If both parents were US citizens and one had prior residence in the US, then there was no retention requirement.
For illegitimate children born outside the U.S. in the same period of time, the transmission and retention requirements are also different.
Another example is that for legitimate children born abroad after December 24,1952 but before November 14,1986, the citizen parent must also have lived in the U.S. for 10 years prior to the birth, but this time 5 of which can be after age 14. The age was lowered from 16. Again, if both parents were US citizens and one had prior residence in the U.S. – no retention requirement.
On October 10, 1978, the laws on retention requirements were repealed. So after that date, everyone who was born a U.S. citizen keeps U.S. citizenship whether or not he or she has ever lived in the U.S. And in 1994, a law was passed to help those who had already lost U.S. citizenship for failure to comply with the applicable retention requirements by providing that they can re-acquire U.S. citizenship by taking an oath of allegiance. 
Of particular importance to those born in the Philippines, however, is the determination that prior residence or physical presence in the Philippines while it was still U.S. territory – from April 11, 1899 to July 4, 1946 – is sufficient to meet the residence or physical presence requirement prescribed by law for a U.S. citizen parent to transmit U.S. citizenship to a child born outside the U.S. This is so even though birth in the Philippines during that period was not considered to have been birth in the U.S.! 
It is quite possible, therefore, that your American mother may have transmitted her citizenship to you even though she’s never been to the U.S. If she did, the next question is whether or not you  retained U.S. citizenship as required. If not, you may still be able to re-acquire U.S. citizenship.