Is it advisable to underline some key points in a statement of purpose? I am an undergraduate student applying to grad schools this year.

I was recently told by my friend (who is already in a PhD program now) that I should consider underlining some key sentences in my SoP, and put them in bold font, since the admission committee most likely won't have the time to read every whole statement, and this would be a good way to catch their eyes on the key points I want to express. It can also expedite the reviewing process so in this sense it is also helpful to the admission committee.

I think this makes sense, but I am still a bit hesitant since I'm not sure if this would be viewed as a poor writing style. Would this be welcomed from the viewpoint of an admission committee?
No.  Never underline anything.  Underlining is left over from typewriters.  It instructs the typesetter to put things in italics.  Nowadays, you should put things in italics yourself.