Agreed, BMI can very occasionally prove be a crude measure and there is an issue (I was undoubtedly in this category when I was younger) of being TOFI (thin outside, fat inside). But with a BMI of 20, I think it's fair to say the likelihood of OP carrying any significant excess fat is low.
And for the vast majority of us - barring bed-bound invalids suffering muscle wastage, or olympic athletes carrying disproportionately large amounts of muscle, BMI is a pretty damn good first approximation. If you have a normal lifestyle (walk a bit every day, maybe do a bit of sport, eat a balanced diet), BMI will be a very good indicator of roughly how healthy or otherwise your weight is (despite the growth of a big industry round selling people fitness or diet products which is geared to trying to undermine it as a measure).
My own experience in my twenties for instance was that doing a lot of weight training for a sport I did seriously (daily training sessions for sport, 3X weekly weights training, a lot of fitness work) was it put about an inch and a half on my chest measurement (obviously no impact on bra cup size, because that's not muscle), and put about half a stone on my weight - which equated to going from a BMI of about 23 to a BMI of about 24. It didn't suddenly magically take me outside of the healthy 20 -25 range. Conversely, the summer I got a bad stomach bug on holiday and lost a stone took me down to a BMI of 21 (and I looked unhealthily skinny) - but still didn't take me outside the 20 to 25 range.
BMI is a good first approximation, and if someone has a BMI of only 20, no-one should be encouraging them to lose weight from that starting point.