For some of them, yeah why not. For others, absolutely not, unless there are extenuating circumstances.
If the champion in question revolves around an air of mystery (
), then unless the reveal is somehow game-changing, learning what's under the mask would actually be detrimental to their character(s). For example (I'm just spitballing here), if Nautilus stripped down and we discovered that the diving suit housed an ancient eldritch being that somehow fused with the former human Nautilus to forge a new, unstoppable incarnation of vengeance (à la Lich King), and they somehow tied the ancient eldritch being into some other, larger story arc, then okay. That's pretty neato. But what if instead we just got some weird necrotic zombie design that didn't expand upon Nautilus' character in any way? Then that would suck. In fact, it would effectively ruin Nautilus' character, because the air of mystery is shattered and nothing is introduced to replace it.
Then there are other champions who don't rely on any kind of air of mystery. 
are two examples. Unmasking these champions wouldn't affect their lore or their character much at all, because mystery isn't a part of their identity. If we learned what Zed actually looked like under that helmet of his, what would change? Pretty much nothing.
On the other hand, for a select few champions, unmasking could have some really amazing storytelling potential. Think about 
's more human side. And what about 
? What was he like before his conversion? Exciting stuff.
Edit: For the record I would count 
in the first category.
For him,it's possible that we may one day see his face.
