It really depends on what you mean by "ground" and "outdoors". If you intend to play on something like a level porch your solution will work fine.
Stability of the playing surface is very important. A wobbly table can absolutely ruin the mahjong experience (at least it does for me). I use sturdy wooden tables with detachable legs (takes like 30 sec to set them up), but they are difficult to transport.
If you intend to play on uneven ground, like when going camping, I suggest to use mahjong cards instead. Now I know that they don't offer the same level of fun as tiles, but they are so much easier to transport and you can play even without chairs as long as it not too windy. The cards are better suited for Chinese Mahjong, since the order of discards doesn't matter and you can just place them in one stack but I'm sure you can make it work. They certainly require less space than a regular mahjong table.
Maybe a smaller travel set like this might work as well, but I have absolutely no experience with that. (Apparently they are tough to come by and are not really worth the trouble, according to some reviews I found)