You do not have to be nice
You noticed that your game is for mobile, so I guess the main question is why nobody downloading it?
FROM A VIEW OF USER/GAMER... IMHO
!!! For whom this game?
1) You write: "Tower Defense" Ok, so if someone likes and look for a Tower Defense this game can be possible the choice...
2) But OK, maybe it's a kind of brain logic game, so the bad graphic not a problem, many trivia, logic and even platformers etc games have a very minimal graphic. So maybe the main target audience is a brain game fans.
But why for the sake someone will download especially this tower defense or logic game? From the first look: Icon looks terrible, the promo images from Google play looks childish and bad. The first thought on a game page is a "another childish crap on a Google Play" And maybe the gameplay is OK, but just compare the first look against the other Tower Defense or Logic game and the answer will be clear.
P.S I didn't play the game. I just wrote what I think.
P.P.S sorry for my English and exaggerated criticism