Is Kintoki a good target for Palinogenesis?

I just want to tell you, that 3* servants gain more stats from grail, but it took more grail to reach 100 of course. There are some people that grailed Sasaki Kojirou, but of course even in lv 100 their stats still lose by far compared to 5* servants.

Gameplay wise, 5* servants usually gain a little stats (from 90 to 100). Usually people grail because :

1. They love the servants, regardless of the gameplay meta
2. Those who use high ATK servants such as Jeane Alter. They grail + MLB their CE, that's why they have absurdly high damage.
3. That servant (which they grailed) is the strongest they have
4. For the sake of coolness and FP generator

So... It's up to you.

I'm not completely sure if what I'm about to say applies for the extra levels from grailing but I recommend checking the growth curve for characters you're looking to grail as that also may affect the stat gain from the extra levels. For example, a character with a reverse S growth curve would gain more stats from grailing compared to a character with an S growth curve.
https://grandorder.wiki/Leveling

Edit: Actually I just checked and when you grail it always follows a linear growth curve.

There's a really good reason why we just refer to that as 'grailing'. In any way 5* aren't preferable for grailing due to them getting less levels 5* gets 2 levels while 4* and 3* get 5 levels, and my opinion about it is if you really like the servant you Grail him/her. Well Kintoki is a nice rider and the best single target rider right now since he's probably a higher NP, but It's your choice if you want to make him your main rider

Gameplay wise, yes. He's the best four star rider in the game, if not the best non-five star rider. Since he can spam NP, having more damage in his NP helps with his consistent damage. In my opinion, if you like him but not really a husbando material for you, just level him up to 90 so he can be on par with five star servants.

rider kintoki is definitely good to grail, most crit-riders are good to grail, as are most berserkers and lancers