Ive been saving a 100% squirtle

How do you lower up a Pokemon?

Blastoise is unlikely to ever be relevant even with Hydro Cannon. Of course, if you really like Blastoise and have more than enough stardust, feel free to power up!

It is highly unlikely it is going to be useful, but 100% are always nice

In terms of how good it'll be, I would say no. Hydro Cannon will make it better than it is now, but from what I can tell Blastoise will still not be a great Pokemon.

But if you have dust and candy to spare, I suppose powering it up a bit is okay. It will probably not be a Pokemon you will use much (unless you have no Gyarados, Vaporeon or Kyogre), but a 100%IV Pokemon with a Community Day move is always nice.
Though I would save the dust for other Pokemon or possibly trading :)

Honestly, why not, though? It'll still be of some use as a Water attacker.

I like Mudkip a lot, I picked him as my starter back when I played Emerald. When I caught a 95% in the wild, I powered him up from level 27 to 35. I use him regularly alongside my Vaporeon and Kyogre.

I don't see any harm in powering up 100% pokemon if you don't have more pressing needs. You can always use them to take down gyms. I have a 100% Warturtle I will probably evolve and max out while I am trying to acquire rare candies for legendaries.

If you are short on stardust then I it is better not to or maybe power up till level 30 or 35.
If you have plenty of stardust why not since it is a perfect IV and if you like Blastoise.

Personally I am swimming in stardust and I am going to max 1 of the best of the gen 1 starter so that I can relive my Gameboy playing days and have a lv99 of each starter.

Only if you're a badass

Vaporeon would still be better than Blastoise