I used to work retail, so I understand how grating it can be to get this kind of question repeatedly when the truth is something you shouldn't say. If you don't want to lie, then you can say something like this:
It can be tough, but I'm getting the hang of it
I'm happy to have any job right now
I can't complain
If there's something specific you do like, you can also answer with that:
People have been welcoming
Management seems pretty reasonable
Especially if you're being asked by co-workers instead of management, they likely don't expect you to love your job and they probably hate their own job a lot of the time. In addition to the typical unpleasantness of retail, customers are being extra obnoxious right now, you have to worry about getting sick, there's more work because of all the extra cleaning needed, etc. It's a rare person who loves their retail job even on a good day, so you aren't alone.
I wouldn't complain at all to management, as the one thing they seem to object to the most is someone seeming grumpy. Whatever your response is to them, give it with a positive and upbeat attitude. They don't really expect you to love your job either, but they do expect you to act happy while you're at work.
In general I wouldn't suggest giving criticism while you're still really new. You can ask why things are done a certain way if it seems strange to you, and maybe ask why they don't do it some other way, but you should do so with an expectation of increasing your understanding, not of changing their mind. Make sure you have a good understanding of how everything works and have shown yourself to be useful before you start suggesting changes.