Of all theories created by this fandom, "Croc is Luffy's mom" is actually my favorite of them.
The reason I have some belief in it, is because after Crocodile learned about Luffy's connection to Dragon, his entire behavior towards Luffy changed.
Before this, Luffy stopped Croc from attacking Whitebeard, so Crocodile should have had no interest in saving Ace, let alone saving Jinbei and Luffy later. It's Crocodile, he's a selfish bastard.
Why would he save the kid who beat him and ruined his plans, then prevented him from fighting Whitebeard?
Even if Crocodile isn't Luffy's Mom, I think Crocodile is connected to Dragon, and gained respect for Luffy once he learned on his lineage.