I think it would a pretty common occurrence for elves (or anybody, really) to drastically underestimate Legolas, especially elves from other realms. Which genuinely is not a problem for him, in fact, he wants it that way. It's just easier.
In the face of his incredibly bubbly personality and open ended curiosity, it easy to forgot where he came from and who raised him. His laughter seems to bright and endless that they forget that very few high ranking Greenwood warriors survived most of the third age. They sense his light heart and gentle soul and forget he's been looking a great evil dead in the eye since he could practically walk.
But then something happens. Perhaps he loses his temper, somebody he loves is injured, or any otherwise sizeable threat appears.
And then like a cloak, the Legolas everyone knows and loves drops away.
The power passed to him through DNA and that which he had collected, learned or earned himself crackles in the air like electricity. His normally calm and peaceful eyes turn colder than steel left outside in the winter. His normally melodious voice drops to be nearly dripping with authority and confidence. Every bone in body suddenly seemingly made of stone. When he looks to you, it's through you. Taking apart every piece inside of you and inspecting it closely, before putting you back together like the world's easiest puzzle.
And it's then that all the elves remember collectively exactly who they're dealing with: Prince Legolas of Greenwood the Great, only son and crowned Prince of King Thranduil, last ElvenKing east of the sea. Grandson of Oropher Of The Awakened, and close kinsman of King Thingol. Captain of the Greenwood Archers & Definitely Not Afraid of You.