MechaGodzilla being created by alien or human in Toho New Series

I mean... in what context? It's clearly worked in both ways.

^I agree. Are we talking abut a Lengendary MechaGodzilla, or a Toho MechaGodzilla?

In the Legendary universe, MechaGodzilla should be like Kiryu, and made out of the bones of a dead member of Godzilla's species.

If this is pertaining to the Legendary series shouldn't the question be: Should Mechagodzilla be created at all?

No, MechaGodzilla should not be in Edward's current Godzilla films. Now, if Legendary were to go into a new fantastical direction later with a reboot, I'd go in a Marc Cerasini novel-like direction.

The U.S. and Japan allied Kaiju force build Mecha-Godzilla, Russia builds Moguera, and wind up teaming up with Godzilla to battle the terrorist weapon Mecha-King Ghidorah. It sounds out there, but it worked nicely in the novel "Godzilla vs the Robot Monsters." Also includes MG vs Baragon and Moguera vs Anguirus.

I think Mechagodzilla can be pulled off in the Legendary series, and I'm sure Toho would be more than willing to sell the rights to Mechagodzilla to Legendary. If that does happen, I'm sure he'd be a human creation. The only way I could see him being alien at all is if he shows up unexplained on Earth under the control of an unseen alien race. But I'm hoping Legendary makes King Ghidorah an ambiguous alien creature, so using the same concept for a giant robot in the image of Godzilla wouldn't really fly. I hope Mechagodzilla appears in Legendary's series, but I hope they use him in a way that doesn't compromise the series' tone.

^ If Mechagodzilla is manmade, there's really no way it could not clash with the Legendary series' tone. The whole idea is supposed to be that the monsters, like the original Godzilla, are forces of nature that we can't possibly hope to defeat, so having a big manmade robot kick Godzilla's arse, or even put up a good fight, wouldn't really fit... unless it doesn't stand a chance against him, which would just be boring. Aside from that, Legendary Godzilla has so far been more helpful than hurtful to humans, and it's even implied that he may be slightly sympathetic to us, so it wouldn't make much sense for us to make a giant robot to kill him.

If Ghidorah turns out to be alien, though, then MechaG could work as a tool of alien invaders. They could have some kind of technology that actually can stop natural disasters (or cause them) to sort of show that they have ascended beyond the forces of nature that we are helpless against, basically preserving the tone of the series. I still hope they just have a new monster for Godzilla 3, though.

The OP, though, is asking about the new Toho series. I'd say the next MG should be alien, since I'm hoping the next Japanese Godzilla will also be unstoppable to humans.

I think a man-made Mechagodzilla could fit in nicely with Legendary's tone if done correctly. The biggest issue is justifying how humanity could create a giant super robot in Godzilla's image. The setup for why he is built could be something like this. After repeated monster attacks in recent years, human civilization is devastated and the world is fearful because they have no weapon that can harm the monsters. But the governments of the world have combined their resources to produce a weapon that can protect mankind. After discovering the skeleton of a Godzilla in the Philippines, scientists have the framework of the perfect lifeform suited for fighting off giant monsters. Using the skeleton as a frame for the machine, scientists create a giant robot resembling the living Godzilla. It is designed to fend off attacks by the giant monsters that have been appearing, and to be a trump card in case Godzilla ever becomes hostile to humanity.

This could give a logical reason for why humans would build a robot Godzilla, and why it would be based on Godzilla's form. It could also be thematically significant, as it is an attempt by mankind to play God and counter the monsters that they have so far been unable to phase. This is a man-made monster, unlike the ones seen so far. I'd be interested to see it end up fighting alongside Godzilla against the final enemy of the series, be it a new monster or something extremely powerful like Destoroyah that cannot be conquered by Godzilla alone. In the end, Mechagodzilla should end up being destroyed to show humanity that they cannot create their own monsters and prove once again that Godzilla and other kaiju are beyond human control.

interseting idea with man trying to play God, personally though, i think a sort of fusion of mecha g and orga would be excellent for that, have him  be a failed Godzilla clone with some sort of computer brain that connects to the nervous sytem to control the monster, kind of like a reversal of how an advance prothestic limb could work. also it could look really, really cool