I think how much you risk randy Usmash is down to your values as a player more than anything. There's pros and cons. In his most recent set vs Hbox I noticed Axe seemed to just outright refuse to ever throw Usmash out in neutral, and I think it did him more harm than good.
The thing is, yeah, Pika's Usmash doesn't have the best range, meaning it's more of a hard read than something that is pretty reliable to hit if thrown out given the opponent is roughly where you think they'll be (Fox Usmash)...but if you don't use it at all then you are basically saying to the Jigglypuff player "Hey, play as brave as you want, because no matter how many times I hit you, you'll only die at 120%+ to a Nair anyway! All you have to fear from my punishes is eating 12% from an aerial!".
Pikachu has such good frame data that he rarely gets rested even if he whiffs something, and he can't be punished that hard by Jigs via combos/grabs etc like fastfallers can, so I think it's completely reasonable to throw out a couple of up smashes per game just to show that you are willing to do it. It has a huge amount of scare factor to the Jigs player and forces them to play more meakly. I think that's worth it even if you don't hit the Usmash you try. At worst you will probably just eat a few Bairs or something. This MU will usually see a lot of neutral game wins on either side before it's over, and that gives you some leniency to lose neutral a few times if what you're doing influences their playstyle in a way which makes winning the rest of the neutral exchanges easier.
Not using Usmash vs Jigs is like playing against a heavily-crouch cancel based character like Peach/Samus and never using Dsmash. Sure, you might get punished for randy Dsmashes occasionally, but does that mean you shouldn't do it? The overall gain you get from manipulating your opponent into playing in a way in which you can beat them more easily is well worth the few times you get punished for it, imo. You need to show them that they have something to be afraid of.
Again, I think it comes down to your values as a player, though.
And yeah, weak Nair to Usmash is super amazing and is absolutely essential learning for every floaty MU. It even works on Sheik and Marth at higher %s. I get a lot of shock from Peach/Puff players when they die to Weak Nair > Usmash > Thunder from a Nair at 40%. I'm honestly kind of perplexed as to why I never seem to see other Pikachu players do it.