How does the speaker feel about the grain the mouse steals?

The answer to your question about how the speaker feels about the mouse stealing grain is found in these lines:

A daimen icker in a thrave'S a sma' request;I'll get a blessin wi' the lave,An' never miss't.

Here, the speaker identifies the theft as one ear out of twenty-four, which he considers a small crime. The speaker is sure that what is left is more than enough for himself.
Elsewhere in the poem, the speaker regrets how his plow has disturbed the mouse's nest. He recognizes how much labor that represents for the mouse and acknowledges that with the cold winter coming, there is no green grass left for it to build a new nest with.
Overall, the speaker feels that the mouse is better off than he himself is, because the mouse lives only in the present, while the speaker can both look backward with regret and forward with fear of what may come.