It's hard to say. I have Super Mario Land 2 on Game Boy, but I'm not sure if I received it before or after playing Mario Kart 64. I definitely remember Wario from the original Mario Party though... played that game way too much!
It was Super Mario Land 2 for me, but I don't recall having any sort of special reaction to him then. I think it wasn't until the Nintendo 64 came out and he started appearing in all of the party games that he really caught my attention.