You mean mountains like those in oldschool SNES games like Breath of Fire?
Atm, those 'mountains' look like walkable rocky floors to me. I'd be surprised they weren't walkable if I were playing a game with these tiles.
An interesting technique could help your square-looking tiles stop accentuating the grid -- try making the 'corner' tiles in your transitions more polygon-shaped, making your landmass more diamond-shaped rather than simply rounded or squared like in the bottom left tile of the water-to-sand portion of your tiles. Each corner tile would resemble a polygon/triangle in shape more than a square or circle-shape. This will make your grid fade a lot more without any additional tile variations.
If you want Breath of Fire type mountains, the way these games made their mountiains (and many other types of tiles) is that they drew a single mountain on, say, a 16x16 tile and placed it in four places on a 32x32 tile (topleft to bottomright) and placed the mountain they drew separately in the middle of these (with the bottom mountains in front of the middle mountain). Very easy technique since, on the corners, they would just omit the respective corner mountain to give it that 'diamond' shape I talked about previously. Hope that gives you some direction.