Let's assume I can react to dash attack.
Don't even start with that word. Average human reaction time is 16 frames or 265 milliseconds. Your average reaction time would need to be less than half the human average to reach to MK's Dash Attack.
http://www.humanbenchmark.com/tests/reactiontime My fastest is 197 ms or about 11 frames which still isn't even close to 7 frames.
You can "predict" MK's Dash Attack, not react.
Don't discount just how hard it is to keep MK out, getting in is what this character excels at. I know Olimar's a defensive character but I also know his wall is far from perfect and MK has the tools to slip through the cracks. MK's Running Speed & Air Speed were actually increased from Brawl as well as his fall speed being changed to fast-faller status, the 12th fastest faller in fact. MK's mobility in this game is in the same league as say Fox, Falcon, Sheik, & ZSS; it's part of the reason we Meta Knights compare him to Falcon since the similarity in play-style and tools is pretty obvious. In fact Falcon's Dash Attack and Dash Grab have the exact same properties: Both Dash Attacks come out frame 7, they cross-up on shield, they're largely disjointed in the front, and they pop the opponent into the air for true combos while both of their Dash grabs travel far, carry their momentum into a slide while they're holding their foe, and can slide off-stage into an auto-grab release for a gimping opportunity via aerial or footstool. The main difference in their Dash Attack is that MK's has less ending lag. ZSS has the same Dash Attack as MK & Falcon, but again MK has less ending lag. Tack on the fact that it, unlike Yoshi's or Rosalina's Dash Attack, sets up for combos until well over 100% damage and it is no stretch to say that MK has the best Dash Attack in the game bar none.
Even with a Pikmin attached to MK, Olimar's F-Smash at best clanks with Meta Knight's Dash Attack and I'll probably just throw out another one immediately or tap my Frame 3 D-Tilt. To keep MK out, Olimar has to predict perfectly. He needs to shield or grab his Dash Attacks, attack or grab his Dash Grab, and predict his tomohawk punishes (being a fast faller with 6 jumps makes it that much harder for Olimar or any character to avoid.) It's a guessing game and MK gets at least 25% + stage control every time he guesses right while Olimar more or less just resets until he can KO. If Olimar whiffs a move predicting MK to move, that's a punish if the MK knows what Olimar's lag frames look like. He's no Brawlimar, those lag gaps are real. If MK guesses right 5 times, that's probably Olimar's stock gone; it's the same for MK too but MK's momentum advantage state is notably more dangerous than Olimar's in this MU and that's a lot of how MK wins this MU. The other part is the off-stage game but I don't think I need to into much detail there. MK's B-Air is a multi-hit strong KO move in this game so, uh, enjoy that.
Meta Knight is forced to play neutral like a low-tier (he's either in or out with almost no in-between) but he punishes like a top-tier.
If you nair with a Pikmin on you hitting the Pikmin first my fsmash will then beat your nair. It is a way for Olimar to creat a sort of artificial priority.
That's a good theory but if you checked the frame data in the OP, you'd notice MK's N-Air has two different hitboxes. The 1st would kill the Pikmin, the 2nd would block your Smash Attacks. I know this from experience. The same goes for Mach Tornado. Additionally, MK's not particularly threatened by a Pikmin latching on when he's platform camping, especially at high percents when it just means he can kill a Blue Pikmin or shield and kill a Purple Pikmin for free or predict your approach and throw out a Dash Attack, Dash Grab, or D-Air. At no point are thrown Pikmin going to do enough damage to matter if I'm camping or waiting nor is Olimar fast enough to run in and punish a FH N-Air (which I can mid-air jump out of) at the range MK will be sitting at. A Pikmin will be lucky to get 2% before MK disposes of it with FH N-Air and I'm always wary of white & purple pikmin in Olimar's line so I'll likely have a N-Air or shield ready for them. If MK's ever close enough to Olimar for Oli to punish attacking latched Pikmin, he should already have a move out coming for you. Again, MK is either in or out and he's very fast on the switch
Finally his dsmash can clip under the ledges of stages, and even a bit in front of them. This means he can safely stand on stage near the ledge with dsmash to get free edge guards.
Again, a nice theory but MK's Shuttle Loop deeply pokes through the stage and would either beat the Pikmin or hit Olimar. It's also not a good idea to charge Smash Attacks against MK...period. Dimensional Cape turns charged Smash Attack into a death wish and trust me, Cape Slash out-ranges Olimar's D-Smash. Its hitbox is kinda stupid tbh, it just hit everywhere around MK for no reason.
Anyway, other than that, I said my piece in the MU General thread. It's a fairly even MU on-stage so long as MK plays Olimars game because he plays it just as well. MK keeps this a 50:50 on-stage but the off-stage and juggle game is where it can shift into MK's favor and MK's KO moves aren't situational. A B-Air's a B-Air and it will KO at 90%, an Up-Smash is a Up-Smash, and F-Smash is a stock.
In my opinion it's
50:50 
at worst or
60:40 at best. Neither character should sleep on the other's defensive capabilities.
