Biologically you're more likely to look like your grandparents than you are to look like your parents. "Looks skip a generation", as the saying goes, and there is some truth to that. Take the Cake Twins for example. At first glance, someone may assume that Mrs. Cup Cake had some explaining to do given one child looks nothing like Mr. Cake --or her for that matter-- and both are not Earth Ponies. But through biological science, it makes perfect sense when explained.
Changelings are a different story, in a way. Creatures primarily spawned by one brood-mother, they all tend to look nearly identical in their formative years. Black, blue eyes, crooked teeth, oddly slotted wings despite only just being born; the whole nine-yards. Save for later developments in their life-cycle, you wouldn't be wrong to say Changelings are all the same on a surface level.
Granted, both Changelings and ponies alike can be born with unique traits that set them apart. A changeling could be born with lavender or red eyes--even heterochromia--, a pony can be born with vitiligo or missing a hoof; sort of 'defects' at first glance, but actually unique quirks that simply happen through the process of generational growth. These things may be strange at first, but they're perfectly normal and can happen to anyone. It is one of the great spices to everyday life, and should be celebrated, not ostracized.
This all said, sometimes biology, quirks, species-differences, and unique chances in life cannot answer the hard questions. Questions that tear at the mind and threaten to fracture the very fabric of someone's world view.
"Sis, why don' 'ah' look like da other kids?" Innocent as ever, Pip looked up to Sunset for all the answers.
She knew this day would come... Surrounded by hollow black caverns, love-devouring insectoids, and two parents of vastly different species, she thought it would have come sooner, however. "Well, Pip, there's no easy explanation outside of ripping the bandage off. But, since dealing with a crying fit isn't how I want this day to go, let me try to explain."
Pipsqueak scooched in close, hugging close to Sunset's leg to hear this great wisdom. "You see... When a mommy and daddy love each other very much, they have a baby. But sometimes they didn't think that through, so they give the baby to a mommy and daddy who did, but never got around to step two. Or daddy couldn't finish step one. Either way, we call this 'adoption'."
For a moment the young colt pondered these weighty words. Pip's young muzzle scrunched in thought, turning things over to one side than the next before he spoke up again. "Does that mean ah'm adoption?"
"The active past tense is 'adopted', but yes," a brief drop came to Pip's face as the implication sunk in, "Buuut... So am I. And I'm pretty great, right?"
"...Yeah..."
"Aaaand the queen's pretty great, right?"
"Yeah," Pip's mood flew up as a broad smile made its way across his muzzle, "And Cap'n's great too!"
"Let's not get too ahead of ourselves."