The RTP was created to be used, so I'm fine with it. Especially for a free game, I would prefer that you focus on the gameplay itself so that the game is fun to play. Though if you're putting it in a commercial game, at the very least use it creatively. You can do wonders by doing simple edits (read as mix-matching) of tiles from the RTP. Things like making a tree with a flower bed below or a table with assorted things are pretty simple yet are a huge breather from seeing all these generic plain trees. TBH, as a player I'd rather play a generic looking but good game than a fully custom (graphics-wise) looking but bad game.