whenever youre talking about people CCing a move, its typically in a neutral situation.
Not always. Mewtwo's grounded kit is relatively vulnerable to CC, so it's a good counterplay option in more than just neutral situations. You mentioned fair and nair further down, but those are spaced very differently from his grounded normals and have different functions. Nair comes into play kind of (it "beats" CC by virtue of the different timings at which you could HC it, making punishment windows ambiguous, but good SDI potentially gets them away from the ledge so it's not something you should really use there), fair not so much in the situations where you'd ftilt. Dash grab definitely comes into play, but you have to commit a bit harder for that and it puts the opponent in a slightly different position. Still a rewarding option, mind.
A small increase in BKB and accompanying reduction in KBG would still net modest reward from ftilt in most situations/percents, it simply reduces the ranges at which you're actually punished for using it and makes it more viable in the mid percent range. It's not really a kill move for Mewtwo, so any potential change in kill percents is negligible.
m2s ftilt is not meant to be an approach move, and since there is relative counter play with CCing it, i wouldnt use it in a situation where an opponent is open to CCing options. its like falco. his ftilt is not meant to approach with. however, you can frequently see falcos using ftilt to secure edge guards, or knock someone away at the ledge at higher percents.
Falco's ftilt also has less startup, more active frames, better endlag, no sourspot hitboxes, is complimented by a frame one move that breaks CC at zero, etc. It works within the context of his kit. Mewtwo's has a clear intended purpose, but its properties (of which CC was meant to be illustrative rather than the core issue) limit its use and give preference to other options in some situations where you might expect ftilt to be a relatively important tool.
in terms of comparing to roy and marth, i know its tough to see their ftilts have much better KB data, especially for their range and coverage, but its tough to compare directly to them because m2 is an entirely different character.
Moves do not exist in a vacuum. Their ftilts have similar design and intended function within their kit. You also had no difficulty comparing to Falco previously when he is relatively far removed from any of these characters in design. I specifically chose Marth and Roy because of their relative similarities (and to point out another example of Roy's overbuffed sourspots).
I already discussed factors that offset differences in frame data such as hitbox coverage, Roy's extra range and potent inner hitboxes, etc. To be clear, I'm proposing that Mewtwo's sweetspot ftilt be only moderately more effective than either character's
sourspot hits. Something like 40/70 knockback (numbers out of nowhere woo!) to make ASDI down less rewarding (poorly spaced hits would still be very punishable) and true CC slightly (only slightly) less so, but still an option at moderate to high percents depending on weight.
in terms of his jab, i actually think its really good.
Jab1 certainly has its uses, but it also has enough built-in counterplay to make 8 frames startup rather excessive. Zelda and D3 by comparison also have even more disjoint and range; D3's has the most range of the three while Zelda's has substantial active frames due to its multihit nature (and it's safe on shield). To be fair though, frame 5 is dreaming. Frame 6 would be fine, even frame 7 perhaps. Frame 8 feels like a Melee holdover that's yet to be addressed.
I generally use this move angled downward to edgeguard certain recoveries. It can cover all but the cleanest spacie SideB sweetspots if you're not able to woop to ledge in time.
Literally no coverage below the stage is kind of a problem there. I don't really care to base a move's viability on my opponent playing poorly. While there are a few recoveries for which it's a legitimately good option, he has other options that are better in those same situations so it still ends up being subpar. Its best use right now is setting up edgeguards near the ledge and even so the counterplay is just that slight bit too strong. It's almost there, just not quite yet.
The idea isn't so much to buff Mewtwo as to make a couple of outliers fit more naturally within his kit. My biggest issue with 3.5's adjustments to Mewtwo (as stated when the patch dropped) is that he's an incredibly difficult character to play now, to the point where most people and even most Mewtwo mains lack a working knowledge of what he's really capable of and much of his potential may never be fully explored (not due to laziness, but sheer impracticality/required precision). Removing free stuff like telehover was good and we all knew it was coming; while what's left is still quite potent it's also rather difficult to work with. While 20XX is theoretically interesting, and I do love me some theorycrafting, he could stand to be more accessible. These are just some small, simple changes that fit within that goal. Other potential changes include things like adjusting his jump/fall animations so that he can perfect waveland moving forwards, adjusting Teleport's ECB behavior (animation?) to make AC tech realistically viable (still difficult, but not to the insanity of 1~3 degree input windows as it is right now... maybe ~5 degrees?), allowing him to teleport through platforms he's standing on (why is this still not a thing?), etc. Will he need compensating nerfs? Sure, but if anything making those changes puts the PMDT in a better position for further adjustments because we'll
all have a better grasp on his kit, testers and balancers included. Maybe we'll even put that kit to good use.