Nope. DD1 was a dead ringer for me at birth and now at 3 years old, you can hold a picture of me at that age next to a photo of her and the only way to tell the difference is I had an 80's mullet :/ DH even got concerned when DD1 was a newborn because she had almond-shaped eyes (my little brother has them, I did as a little kid but they've opened up a bit as I've grown up) and he thought maybe the milkman was of Asian descent lol!!
Then DD2 was born and right from the minute she popped out facing the ceiling, she's been DH all over - all she's missing is the doodle! Before she even opened her eyes and started to cry, she looked just like a tiny, almost-bald DH covered in grease lol :/ Once we wiped her down and discovered the strawberry blonde curls, we knew we'd got one of each - a mini-me and a mini-DH. At 18 months she's DH in every single way, from the cheekiness to the facial expressions to the position in which she sleeps at night. So cute!

It is common for babies, whether first or fifth, to resemble their father in the early weeks/months of their lives, but it's not always a guarantee and it certainly doesn't mean anything - DNA is a complicated thing on its own, let alone when it mixes with someone else's to create an entirely new human being! It does seem to be common for both sides of the family to want to 'lay claim' to a new baby by having it resemble family members though so I hope your MIL's remarks are coming from a desire to bond with the new baby as well as she can