No taste?

yes

Yep, taste and smell can change. Ingredients in a product can change Also, the way it is prepared too. Appetites decrease as well, so food is less appealing.

Could be a lot of reasons.
If she is having a problem with allergies, a cold, dental problems. There could have been a change in the product itself.
Try another brand or another flavor and see if that makes a difference.
You can always enhance the flavor of a yogurt by adding fruit, or an extract.

But as we age our taste buds do become more dull. this would not happen right away and if the last yogurt you bought was alright but this one isn't I doubt it would be taste buds on the whack.

This could be because of a product change as well.

Have you tasted the yogurt to see if it is different? My fave yogurt had a period that it was sour for some unknown reason. Maybe the taste mom could enjoy has been changed.

Just a thought.

And there are viruses that can cause you to temporarily lose your sense of taste.

People with Dementia lose their taste and smell.

Yes but usually the sense of sweet is the last one to go. I agree that she needs to be checked for other possibilities, like covid. My sense of taste went completely away for several weeks after I had covid. It could also be a sinus infection.

This loss of taste can happen, but just watch for any other symptoms for a few days. You lose your taste with covid and sometimes there are few other symptoms. If this is the case it will return.

Loss of taste can happen for other reasons as well, such as Lewy's dementia. This would likely NOT be the case here.

Sometimes it is medications, and just temporary. I wouldn't overworry it at present.

Yes, taste and smell but that's rather sudden, I'd watch for any covid symptoms too