is this a bad opening idea? d4 c5.

Against d4, I often played c5, if my opponent move c3, then I have this plan:

1. bring the light square bishop outside the pawn chain when i push e6. make a small escape square by h6.
2. if the opponent took their dark square bishop out, put queen on b6 to pressure the b2 pawn and support the c5 pawn by eyeing the d4 square.

I thought these are good ideas, but the game reviews and analysis kind of against it. 
here is the full game (I am black).

Not a bad idea, just some ideas for White:

1.d4 c5 is a sub-par way to play a Benoni (after 2.d5!).

In general the Benoni structures are OK after white has played c4-c4 because this square isn't available for a knight, or bishop.

by playing c5, if white takes, they lose a center pawn for a flank pawn when black plays e5 to open up the bishop

That is just the Old Benoni. A better way to play the Benoni is d4 Nf6 c4 e6 Nc3 c5

The current trend is 1.d4 Nf6 2.c4 c5 3.d5 g6 followed by ...Bg7,0-0, d6 and ...e6 later on. This avoids a few annoying Benoni lines, but allows White to take back at d5 with the e-pawn.

It's thought to be weak by black. Recapturing with the e pawn allows white to dominate the e file. White leaves the c pawn at home unless they want to transpose into a regular Benoni line, so white is a move up, development-wise and also has a stronger postion.

Good one