Monroe's fondness for Alma is palpable in the grocery store scene, but I don't think the preoccupied Alma has caught on to that yet. Her look of horror when the jars tumble and regret when Monroe surveys them indicates that he is like any boss to her at that point.
The character of Monroe is played to be pretty transparent and straightforward — a stand-up (if comparatively boring) guy. And you can tell from his 'delayed' reaction to the fight at Thanksgiving that he's a little bit afraid of Ennis. So if he & Alma had had anything going on by the grocery store scene, it would have shown in the way Monroe greets Ennis: either a little guilty or a bit afraid.
[A wife/mother working outside the home] could even be considered faintly shameful . . . an indication that the husband wasn't capable of supporting the family himself.
I think this is part of what vexes Ennis, especially in the 'bucket kick' scene. Again, I don't think Alma has anything going on with Monroe here — she's certainly not 'gussied up' in any way, nor particularly giddy about going to work. This is shown right after a J + E fishing trip, indicating to me that her resentment of
this (channeled into passive-aggressive behavior relating to finances) is fraying the marriage.