Your mother was spot on.
Chiles, and specifically, dried chiles LOVE salt. The flavor “blooms” when the chile/salt ratio is right.
Way back in 2002 (or maybe it was 2003) I did a weeklong series of classes with Rick Bayless and Ricardo Munoz Zurita (Google him if you don’t know who he is) offered through the Culinary Institute of America, Greystone. I think there were about 20 of us all together. Rick generally did a class in the morning and evening and Ricardo in the afternoon. I remember the first session. When it came time to season whatever it was he was making, Rick added salt, then added some more salt, then added even more salt. You could see most of the people in the class beginning to blanch at the quantity they thought he was using. Even I thought he’d gone over the edge, but when we finally got to taste, it was perfect and not salty.
Ricardo explained why the salt was necessary later that day and they tag teamed that particular message the rest of the week. Other Mexican chefs - professional and home - that I’ve had the pleasure of taking classes with have also hammered home that message. Chiles need salt to reach their full flavor potential.
There is an easy experiment you can do to see for yourself. Take a few chiles, 2 or 3 guajillos and a chile de arbol, seed and devein (or not if you like heat), toast, and soak. Drain and put into a blender with a little fresh water, enough to blend to a smooth, pourable paste, about like Heinz ketchup. If you don’t have a high speed blender, pass the chile paste through a fine seive to get rid of any skin or other residue.
Put it in a bowl and then taste a little. It’s not going to taste very good. The flavor will be flat and possily even somewhat bitter. Then add about a 1 teaspoon of salt and mix well. Taste it again. You should be a noticeable change in the taste, but it still won’t be great. Add a 1/2 tsp more, mix and taste. The flavor should improve again Add another 1/2 tsp, mix and taste, it should be significantly better, but you’re not there yet. Decrease the salt to 1/4 tsp and continue adding, mixing and tasting. At some point you will reach the right balance and the flavor will “bloom” and you should be able to really taste the difference. It takes way more salt than you think, so don’t be timid. When the flavor is balanced, it should be full and round in the mouth.
I’ve also found the chiles need salt idea is necessary for salsas that aren’t table salsas, such as mole or pipian. Adding enough salt brings their flavors to life as well.
Sorry this is a little long, but we Americans have been taught that salt is not our friend and to be wary of it. It’s a vital ingredient in Mexican cuisine and not excessive