Pika main coming in here. This MU seems infuriating but I can't see it being much worse than 50-50 unless the MK main I play is just bad at the death combo on Pikachu specifically [and he death combos a lot of people, including some of the other characters I have quite reliably... but he can't seem to quite kill Pikachu with uairs -> shuttle loop except on Town and City].
Pikachu can edgeguard MK with a few tools, but it's tricky at best. [Fastfall (optional)] bair can often interrupt an up+B and drag MK back downwards... if he misses a tech it's usually a stock, but if you get sent away, Pikachu can try to QA back to ledge and immediately drop bair again, and usually will fastfall that one late to suck MK down, which can gimp him if neither bair relied on the two-frame window. Pikachu's dair also lasts an amazing 13 frames and has only a sweetspot, and it seems to have a solid disjoint [it's traded with Cloud's uair and one Sheik main claimed Pika dair beats or trades with Sheik uair], making it somewhat safe to either attempt a snapback [the two-frame ledgesnap punish] or just stop the shuttle loop cold. Pikachu has to rely on thunder jolts or thunder [or maybe a godlike dair] to stop drill rush once it gets going, but it's often somewhat obvious to fire a jolt when MK is in that position, or contest the startup before he even starts moving.
On the other side of edgeguarding, MK can attempt a bair edgeguard on Pikachu's QA, and while there's always a basic 50-50 with aiming at Pikachu's recovery [do I go up and in or in and up?], it's strong enough to kill rather early. That said, if you whiff [or worse yet, fastfall to try to cover both options and whiff], that leaves you in a position where bair and dair become effective at hitting MK, if not gimping him. Nair is alright at stuffing Pikachu, especially near the ledge, but Pikachu will often be in a position to simply aim for onstage OR the ledge, and that's what makes stopping him so tricky, when he can often fastfall and then go straight to ledge if you actively attack where he is. However, if you catch him once and he has no jump left, it gets easier [but still hard].
That said, onstage seems annoying for both characters - we both have very safe moves [MK's dash attack is really really safe, but Pikachu's dtilt is not punishable on block unless you can shieldgrab it [up+b is too slow], and it can definitely be spaced to avoid that] and high movement speed, while Pikachu's Quick Attack can be used almost whenever to retreat, attack, or regain stage position after a trade. Although I have to test it, I think it's possible to SH dair over MK's fsmash and hit him [you can start the SH outside of fsmash range, be out of range vertically as you move toward him, and hit him before his fsmash can hit you], but I can't confirm that at the moment [I just know it's seemed to happen before]. If that's the case, Pikachu may have a somewhat reliable punish to fsmash [besides just throwing thunder jolt and forcing you to fsmash the jolt, or get hit by it if it was aimed at MK's head and you're slow to release it]. Pikachu's fsmash should outrange MK's, so if you see a Pika start charging fsmash, yours won't reach to punish - either jump over it or wait to dash in after it ends [it has lowish endlag, but it's nowhere near as fast as MK's dash attack or fsmash].
Pika's only kill confirm I know of that truly works is landing fair -> usmash when Pikachu has high rage [it should true combo based on weight and some other stuff I believe - K Prime made a mini-guide on it at some point], while MK has a lot of kill options, but Pikachu is safe enough to where they are probably hard to land, and I'm not sure how effective shuttle loop stuff is on Pikachu [as I mentioned before, an MK I know proficient at taking stocks with MK very quickly can't seem to death combo Pikachu very well if at all... it's almost like Pikachu becomes a super heavyweight in the MU or something, but without the drawbacks associated with very high weight]. Pikachu is still quite light though, so any fsmashes MK lands will send quite a way, and as fast as Pikachu is, that move is still very hard to punish at all, never mind with a KO move [unless you just fsmash the wrong way].
It would seem about 50:50 to me - Pika isn't that easy to hit and can frustrate MK in neutral sometimes, but MK has some fierce combos and more KO options/setups that are safe(r).
Edit: Dropping back in to note that the death combo can work, but apparently the percent window for starting it is pretty tight. I don'the know what that window is, but it'seems part of why Pikachu seemed so hard to death combo - it's supposedly a small range to start the combo.