There's nothing wrong with being foreign born, as long as you came here legally. So please be very precise when talking about legal versus illegal immigration, which btw, is not something the left, nor the media will do.
With a ballpark total US population of 338 million, saying "15% of the USA is illegal immigrant" would mean over 50 million of the people here are illegals, so the number (15%) certainly seems high.
otoh, IF you're saying 15% of the immigrants each year are illegals, then the number might seem low.
It's all in how it's said and what it actually means.