That's because you're using the word "saint" in a different sense to how it should be used as explained below:
1. The word "saint" in a more colloquial sense, as you're using it - eg when you say "someone is a saint" or "someone is no saint" suggests someone is perfect or has led a sinless life.
2. The word "Saint" (with a capital "S") in the Catholic church is an official title given to certain people whom they deem worthy of canonisation or beatification. There are strict conditions to meet in order to have this revered status, but it doesn't mean they have to have led a perfect or sinless life. Although people do mistakenly tend to think they have led such a life, which gives rise to sense 1 used more colloquially. Now whether these people are actually worthy of beatification is another matter which I don't want to get into here at all. But long story short, I don't agree with this title and don't think it should exist.
3. The word "saint" in the Bible is the Greek word "hagios" which translates as "holy" and is the proper sense in which this word should be used. However, "holy" doesn't mean that one is holier than others (ie leading a more perfect or sinless life). Instead, "holy" or "saint" simply means "set apart" for God by virtue of one's relationship with Jesus. According to this sense, every Christian is a saint, because it doesn't carry with it a special connotation but the word is pretty much just another word for a Christian. It's not a big thing of the same kind of significance that is placed on the word like the other two senses above.
So in one sense, yes it's true, Mother Theresa was no saint in that she didn't lead a perfect or sinless life (sense 1). But on the other hand, technically she does officially have the title of "Saint Theresa of Calcutta" (sense 2) whether you agree she should have that or not. And on the third hand (because we all have three hands
) she was a saint in the Biblical sense of being set apart for God (sense 3) simply because she was a Christian.
Now whichever sense you want to use, comparing Trump to her is still an insult.
