My thinking on the move being a mistake is that this pawn move is not necessary to develop all your pieces, and a pawn on b3 does not contribute much to the position. Also since b3 typically is used to put the bishop on b2, and since your pawn on d4 would be blocking the bishop it would also not use the full potential of the bishop. In addition to this the move b3 also creates pin on the pawn. It is pinned to the rook initially, and if you play Bb2 then it is pinned to the bishop. You are also ahead in development in the position, and playing b3 allows your opponent to work on developing and closing the gap in development. Developing the knight on b1, the dark squared bishop (Bf4 would dominate the e5 square, and make it hard for black to play e5 with out prep and possibly giving up something in there position depending on how black proceeds before playing e5), or castling would have been better. As far as pawn moves in the position c4 or c3 would be far better then b3. c4 (I would definitely have considered and possibly played this move) controlling the center more, and creating a very strong pawn center. c3 supports your pawn on d4, and makes it harder to undermine your center. Basically b3 does not accomplish or is necessary to accomplish any of the goals in the opening.
Hope this helps.
I made another move that seems to be a mistake, but I don’t really understand why. What indicates that this move is problematic, and how can I recognize similar mistakes in the future to avoid them?
For example, in this case, I played b3 to open up for my bishop. if my opponent plays c5, I go Bb2. Looking back at the position before I played b3, how can I evaluate it to see that b3 would be a mistake?