I dont think that is to much at all. I dont know if you are going to do any mods with it, but everything you add would add weight. My 15 foot jon is rated for a 25hp, i say go for it! You will get to the fish faster.
I stuck a 25hp on my old 14' 1946 FeatherCraft after I repaired/modded it and it was the best thing I could have done for that lil boat. Your boat will not have any trouble handling the 12hp.
If you got the motor or it's already on the boat,I'd go with it.If you're looking to buy,I'd go with what the budget allows.My budget allows for a 5HP motor,plus I can remove the motor and carry my boat in the bed of my truck. :mrgreen:
I have a very heavy 25hp 4 stroke on a 14' Alweld and it holds it fine. We ran the rivers for years
growing up with a 14'L x 36" W riveted hull and 25 hp Merc