I'm currently having a difficult time trying to figure out where to place the return commands to not have the code run through each line, so that the flowers appear 1 by 1. I know that this is not the best way to code this, but working on optimizing it later (if you have additional advice, I'd be happy to hear it!) 
place pink petals under cow if conditions match
execute as @e[type=minecraft:cow,tag=leaf_cow] at @s if block ~ ~-1 ~ minecraft:dirt run setblock ~ ~ ~ pink_petals[flower_amount=1]
execute as @e[type=minecraft:cow,tag=leaf_cow] at @s if block ~ ~ ~ pink_petals[flower_amount=1] run setblock ~ ~ ~ pink_petals[flower_amount=2]
execute as @e[type=minecraft:cow,tag=leaf_cow] at @s if block ~ ~ ~ pink_petals[flower_amount=2] run setblock ~ ~ ~ pink_petals[flower_amount=3]
execute as @e[type=minecraft:cow,tag=leaf_cow] at @s if block ~ ~ ~ pink_petals[flower_amount=3] run setblock ~ ~ ~ pink_petals[flower_amount=4]
execute as @e[type=minecraft:cow,tag=leaf_cow] at @s if block ~ ~ ~ pink_petals[flower_amount=4] run setblock ~ ~ ~ moss_carpet replace
schedule function leaf_litter:15secs 15s