I am playing at normal mode. Does the easy one mean less infestations, more cash? etc..

It might mean that you are more likely to get a MP from buying seeds from the store. I play normal mode too.

i nkow that easy/nomral/hard affects the number and level your plants are at at the very very very beginning of the game, other than that.... sorry

I don't know for sure for Plant Tycoon, but for Fish the level also affected how easy it was to grow the higher level fish. For example, even in a level 1 environment you can temporarily have level 2 fish if you are playing "easy" mode.

I imagine it is sort of the same for the plants.