I guess you got a lot of conflicting replies here, but I tell my observations.
We were there on Mother's Day last year. Mk did not seem any more crowded than it was during the rest of the trip (which was pretty much moderate all week)
The only change that we saw was at World Showcase in Epcot which seemed a little more crowded earlier than usual. We went over there about 2pm.
Ther was also a large crowd at this special Mother's Day brunch they had in WS at the tent pavilion thing they have, that was built for the millenium.
Otherwise it didn't seem more crowded. They were handing out roses to all moms at MK entrance, which my wife thought was very nice.