Power up MoltrYes or No?

No, first the breakpoint also depends on the defender lvl and iv which changes. Second, if your moltres can't beat blissey there are other options.

It would be really bad for your username (It's super effective against insects).

It's not bad to power it up to around level 30 to try it out. With co decay, I think it's foolish to try to hit breakpoints against gym defenders. That said, Moltres is an excellent attacker with the added bonus of resisting dazzling gleam.

If that's all you want it for, then absolutely not. Just use the high level Flareon you assuredly already have.

No, fire Pokémon aren't very useful right now.

Have one fully maxed out powerful mon each

Up to you, I'm going to max my Moltres out, however, I'm not doing that for Blisseys only.

Not all Blisseys have Dazzling Gleam, and Machamp/Focus Blast Mewtwo are much better options.

Moltres is good for general DPS and resisting fighting/double bug and grass is a super plus. It also makes extremely quick work of Venusaur and Exeggcutor Raid Bosses (and it will be around on par with Scizor/Pinsir for Celebi) and some defenders like Steelix. I'm also powering it up to prepare for some Gen3 defenders (Like Metagross, who I would argue can only be truly overpowered by a strong Fire-type. Gengar will take massive damage from possible Psychic attacks, while Tyranitar and Rhydon/Golem are vulnerable to possible Steel-type attacks it may carry)

Plus, it's a legendary that, for the time being, you just don't see anymore. Something like a flareon you'll have all year to gather candy and dust for, but, at the moment, Moltres comes down to once you have enough rare candy and dust...you're not catching more Moltres tomorrow.

Agree there are better options for Blissey than moltres. If you really like moltres orbneed a decent fire attacker then power it up to about lvl 30. That's what I'm doing w Entei and hopefully just relying on the candy I got from catching him (no plan on using rare candy).

Although Fire Pokémon aren't that useful atm, they will be for gen3 I believe.

No. Machamp is way “cheaper” and will will get the job done quicker.