Maybe they were saying that Chappu's personality is so similar to that of Tidus that it makes them seem alike.
I know i've had two friends that acted very similar and it reminded me of them when they talked or had certain actions about themselves.
But, in terms of physical looks, nahhh, I don't see it at all.
No, I really don't think that's the case. In the beginning when Lulu and Wakka are outside talking she says "He does look a lot like Chappu. I was surprised too when I first saw him"
And then when Tidus asks Wakka about Chappu Wakka says, "My little brother, Chappu. He looked like you." If you study the picture that I compared Tidus to with Chappu, they really do look somewhat similar.
edit: Here's another picture to compare him to

