Can a Pokemons happiness go down and if it does how can I see the number?

Question

I love dialga,but I want it to have full happiness For one of its moves but for the same Pokemon the damage is different

Answer 1

The only moves that have their power rely on friendship are Return and Frustration, and I'm guessing you want Return. (Although there are many better moves for Dialga >_>)
If you want it to have full happiness, do the following:
-Give it a massage at the place in Mauville City (if I remember correctly)
-Just walking around with it
-Feeding it vitamins like Zinc, Carbos, etc.
-Leveling it Up
-Feeding it EV-lowering berries like the Pomeg Berry and the Kelpsy Berry.
-Have it hold the Soothe Bell
-Have caught the Pokemon in a Luxury Ball

You can't check the exact numerical value of its happiness, but you can go to a house in Verdanturf Town to check about how happy it is. The statement for maximum happiness is: "It’s extremely friendly toward you. It couldn’t possibly love you more. It’s a pleasure to see!"

Now, to lower friendship, it is much more difficult. To lose friendship you should:
-Use bitter medicine like Energy Powder and Revival Herbs
-Let it faint in battle

And that's about it. This info is from Serebii's page for Friendship.

Answer 2

Friendship will decrease on a Pokemon. Fainting will let the number drop by 1, Heal Powder and Energy Powder will make it drop by 5 and the Energy Root and Revival Herb will let it drop by 15.

And you can't see the exact number, you will have to go to a Friendship Checker which are foubd in Santalune and Laverre. Place your Dialga at the front and talk to the person. They will say a phrase based on how high it's friendship is, which you can decode in the source.

Hope I helped. :)
Source: http://bulbapedia.bulbagarden.net/wiki/Friendship