Is walking in the supermarket a good exercise?

It's better than nothing. I go out for walks and I consider it good enough exercise, I mean you shouldn't go running during this time of year with ice on the pavement. If you walk briskly, you should travel 1.6km in 20 minutes and that alone would burn 80 calories

It's better than zero exercise, but it's utterly inadequate in terms of health and fitness.

Consider walking in a shopping mall if you can't be outside.

Although I'm 70 and overweight, I just shoveled snow, so I view weak excuses with a jaundiced eye. You may not be able to walk a brisk five miles right out of the gate, but if you walk a minimum of four times a week for thirty minutes at whatever pace you can manage, you'll be pleased at how much easier it gets.

It's good for you and quite the pleasure if you let it be. Let's go together.