Most babies and toddlers do suck their thumbs which is very normal and soothing to them.
I refused a pacifier as a baby and sucked my thumb from the time I was born until I was 4 years old.
But then again my nephew also refused a pacifier and doesn't suck his thumb at all but he prefers to suck on a blanket instead.
So it's up to the individual baby and toddler or child as to whether they prefer to suck their thumbs or not but the majority of babies and toddlers either suck their thumbs or suck on pacifiers.