why people down play all blue and say trying to find it is not "kingly ambition"
Because when it comes to Sanji and Sanji alone, he doesn't receive a free pass unlike other characters. People will find their ways to try to downplay Sanji as much as possible. Why? Dunno.
Finding All Blue is like trying to conquer the sea itself. Not only you have to bring your ass out into the ocean trying to look for it which requires immense willpower, you also have to destroys the Red Line which requires strength to do it. And despite all of that, the 4 seas might not even jointed correctly which requires intelligence to prevent any fatal mistakes. Sanji have the willpower, strength and intelligence to find the All Blue. What matters is the time.
Trying to become the WSS only requires willpower and strength, no intelligence involved. Trying to become the PK involves willpower and strength, maybe intelligence too but we all know that Luffy and Roger ain't the smartest characters there is. That's why they needed Oden and Robin.
"But but, how can finding a pond of fishes be considered kingly ambition? No one gives a shit about it."
Yeah, just like how nobody gives a shit about WSS title bar Mihawk and Zoro. At least the All Blue got hints on being linked with the One Piece which is a harder path than becoming the WSS itself.