Yeah, to me, it seemed that most of the characters in Earthbound, especially, just had PSI since they were born. In that game, the only one who gained PSI some other way was Poo, but even then, it's kinda implied that he had some innate talent for PSI before he completed his Mu training, the latter being more of a ceremonial formality having to do with his royal lineage.
Actually, Poo already has PSI before Mu Training. You can try Lifeup (Although Poo will already have full HP). In fact, he has a larger variety of PSI moves than any other character at that level. However, it is possible that his training contributed to his PSI. Poo's PSI abilities are easily the most potent in both EarthBound and MOTHER 3, with the best and most reliable PSI attack (Poo's PSI Starstorm Ω is not only more powerful than Kumatora's PK Starstorm and Ness' PSI Rockin' Ω, but also more efficient than those two and even PK Love Ω), but also every single type of Recovery PSI except Refresh and Lifeup Ω, access to all versions of Shield, Brainshock, and PSI Magnet. However, he has low Psychic Points reserves, which makes it possible that he has this variety of PSI because of his training. PSI Starstorm was explicitly shown to be earned through training. Poo can also recovery Psychic Points by simply drinking water, which can be a result of his training, because no other PSI user is known to be capable of this.
So, yes, it is very possible that PSI users have some natural affinity for PSI, but can increase this power through proper training.
What part of the Mother games weirded you out the most when you first played through it?
Moonside is one of the weirdest parts of the game, in my opinion, what with teleporting men, and invisible ones, too. It does not help that you have to face clocks that can stop time, and the Evil Mani Mani Statue itself at the very end. You are stuck with only Ness and Jeff, and only one of them can equip the Night Pendant.
The Saturn Valley, Dalaam (Especially when you realize that Ness fell asleep after eating "Magic" Cake from a stranger. Does this remind you of anything?) and Tanetane Island, as mentioned, are also up there. Magicant is also a candidate, in both MOTHER and EarthBound.
What part of the Mother series made you laugh the most?
Plenty of candidates for this.
*You read a sign about watching out for falling objects. Suddenly, the photography man falls from above, takes a picture of the party, and flies up again.*
*Kumatora sets off a trap that makes a trapdoor open under the party, causing it to fall down countless stories.*
Wess: PRINCESSSSSSS!!! (Yes, I counted the number of "S"s and exclamation marks.)
Kumatora: My bad.
"NESS NESS NESS NESS NESS NESS NESS NESS NESS NESS NESS NESS NESS."
"The Men's Room Sign tried PK Starstorm!"
*You defeat the Mysterious Metal Monkey. It is a rare, metallic enemy that may run away. It must give out loads of experience, right?*
"It was a good experience." (This equates to absolutely no experience.)
The Doorknob is everywhere. You cannot escape the Doorknob.
Sign: Preserve our beautiful scenery! Please refrain from posting signs.
What part of the Mother series is the most moving?
The death of a certain person who is very prominent in Chapter 6 of MOTHER 3, and died in Chapter 1. Even more than the ending on MOTHER 3, in my opinion. The flashbacks, the music, her presence in Chapter 6 itself, her actions, everything is very moving.
In EarthBound, the cutscene right before Magicant is also very moving, not to mention the final battle. People usually note it for its scariness, but the way you defeat the final boss is moving in itself.