Forced to mask

So I work at Dollar General and my boss loves me because I'm a hard worker. But today she told me that some customers have been complaining that I'm not all peppy and friendly when I ring them up. She's saying I need to come in and be super happy and nice to people all the time. My question is, can my boss really make me mask and be all cheerful and stuff just because some customers think I'm being rude, when I'm really not?