6 foot 170 is on the thinner side unless you have zero muscle. When i was in the army i was 6 foot 175 at my thinnest and my family said i looked unheathily skinny. I feel my best around 190, but i may never get back there
I guess it depends on your frame. If you have very short arms, narrow shoulders, i guess you could be 170 and flabby...but my guess it that is not true, and you are just overly critical of yourself. You probably need to GAIN weight in the right places to fill out. Increase your back chest and thighs, even if you maintain body fat - you'll have a better appearence.