Hola Fernando, I am pretty much in a hurry but yet..
1.- Is there a lot of contrast between the car and the grass?
yes there is, but that's not necessaril a bad thing
2.- Should take away the grass field and getting more mud and earth?
maybe, perhaps deeper tracks traces behind the tank
3.- Should fill the gaps with more figures or something?
no
4.- Are the things that?
"credible" you mean? well, there's something very sweet, Switzerland like about your dio, that must be the very nice little flowers, like I told you once, we are abit in a Communist ideal dream there

Otherwise, think about putting a bit of matt coat on the figs maybe..
All in all, I think this a is very good diorama as far as beginners go. There is no major mistake or anything like that. Inded there is colours and a very nice balance between the elements, really I think you should be quite proud indeed.
What you have to learn now is getting a bit more realistic about the colours and atmosphere, learn to undo the ground cover so that it looks like real nature, maybe blend tanks in their surroundings better etc etc, but you've got time for that!
JB