Before we begin, your indoor mapping is okay. Just thought I'd say that first.
1) 3rd row, 4th screenshot. (all will be counted from the left.)
The grass (note: grass, not bush) needs darker coloring, and a bit more detail, but don't overdo them.)
2) 4th row, 1st screenshot.
The map looks really plain. Add more flowers or maybe a tree or two.
3) 4th row, 2nd screenshot.
That line of green stone thingies (which I'd call ''plant'' from now on) seem really unnatural. Try adding a variety of restrictors like fences, green hedges etc.
Same applies for the 3rd (and 4th) screenshots.
4) 5th row, 1st screenshot.
Looks plain too, add those smaller trees, or even bigger trees if you are confident about your tiling. (By bigger tree, I refer to the 12-blocked versions)
5) Final row, 2nd and 3rd screenshots.
Add more rocks. The cave looks very unnatural.
6) Final row, 5th screenshot.
Way too little details in the in the dessert.
Don't worry, beginner mappers are like that. The thing u must remember is to never EVER get help for your mapping. Advices and tips from others are okay, but don't depend on other mappers or you will never learn mapping yourself. I will VM you a small mapping tutorial my cousin made.
I would like to point out that I am by no means a beginner mapper. I have been mapping for a long period of time. Additionally, I would like to point out that outside of flowers, these maps are the ones that Game Freak has made for Firered and Leafgreen, with slight alterations to make them appear as they do in Heartgold and Soulsilver, with a significant tileset change. Mt. Moon looks like that in both Firered and Heartgold. Also the grass is official Heartgold grass.
I realise that you are trying to help, and I understand some of your concerns, but considering the maps are based off of existing Kanto maps from Nintendo, it is not necessarily my fault that certain things can be considered plain.