Not much luck with the sheep!

I started with 103 Mareep candy, evolved 6 Ampharos, with 3 of them being shiny. One shiny was level 23, 84%. Have about 60 candy left. Highest CP was 2167 and three were just under 2000. I already had a 98% regular level 20 Ampharos, if for some reason I start to care about having an almost perfect Ampharos.***I was busy with family during Venusaur day, didn't get a shiny...I did evolve a 98% bulbasaur I already had and found/evolved a pretty good level 30.

10 shinies, most very poor IVs, but walked around for most of the time in a location that generally has poor spawns. Had weather boost, don't know if that plays into it or not. Had enough candies to evolve six by the end, one of those I decided to have be shiny. Will say, had depressingly poor luck getting my Ampharos to have Dragon Pulse, only the shiny and one other got it and they were my last two to evolve, all the other ones ripped me off (one of each charge move) and wouldn't have evolved them all if I'd known it was going to do that to me. Almost had the same thing happen with Venusaur's Frenzy Plant and it was really irritating; completely missed the cutoff like a fool for Dragonite to get Draco Meteor so can't use that one as comparison for "special move likelihood".

We played for the full duration of the event - I caught 2 shinies and the wife only got 1 (which she caught at 4:50 PM! The same thing happened to her with Bulbasaurs too). Compared to the Pikachu and Dratini days, it seemed this event had significantly fewer shinies...

I had no time to play and caught like 10 mareep I think and was fortunate with 2 shiny!

Easier

The 2nd I caught about 3 minutes into the event was shiny. Turned out to be the only one I got out of around 100 caught.

My luck started out terrible, then got crazy. Zero shinies during the first half of the event, then 18 shinies during the second half of the event. Definitely the highest number of shiny Mons I've caught during a Community Day event so far.

My day went almost the same as yours. I caught 116 Mareep with my only shiny coming with about 8 minutes left in the event.

Mareep caught: ~150-155
Shiny Mareep: 3
Compared to others that day: Most averaged at about 4

You have a bag with 100 marbles, 5 of them are pink, 95 are white.

On Monday you randomly pick out 20 of them and you get 2 pink, 18 white. Then you put them all back.
On Tuesday you randomly pick out 20 and you get 3 pink, 17 white. Then you put them all back.
On Wednesday you randomly pick out 20 and you get 1 pink, 19 white. Then you put them all back.
On Thursday you randomly pick out 20 and you get 0 pink, 20 white.

Hmmm it seems like on Thursday there were fewer pink marbles in the bag. wtf?