Container, or not? Full sun, or not? Fertilizing, or not? Started from seed, or purchased from a nursery? You don't give us much info to go on. Help us help you.
I planted a purchased Chocolate Mint plant last fall near the lake shore that gets morning shade and hot afternoon sun. I don't fertilize it. I don't even water it now that it has a decent root system. It tastes like peppermint.

It's in a container I'm not very good at fertilizing :)but I do water it everyday. It's in the sun, early morning is some shade and about late afternoon it's shade. I purchased the plant from a nursery.

Did you and your niece purchase the plants from the same nursery? It's possible they were mis-labeled as there are many kinds of mints. Can you take a sample back to the nursery and have them compare it to theirs? The only other things I can think of are too much water and/or not enough sun. The essential oils are usually well developed in full sun. Don't worry about fertilizing, they really don't need it. Many herbs do well in poorer, but well draining soil(make sure your containers drain well). Or, maybe your potting mix is too rich in nitrogen, which can cause lush growth and may affect flavor. Could you post a photo?
Anyone else have ideas? FataMorgana? Daisy?

Too much water for a plant kept mostly in the shade. Herbs a little drier have stronger tastes.

FataMorgana

I'm amazed I can over water lol. I'll see if I can remember to get a pic of it. It's actually looking kind of scraggly now compared to how lush it was. And I've noticed a bit of browning on some of the leaf edges and some little brown bugs. I did use Miracle Grow potting mix at least I think that's what ended up in that one and I do have holes drilled into the pot which is elevated so it truly drains.

It isn't about the drainage. It's about too much water in the plant and leaves diluting the taste. There is no reason to water every day unless you are having a time of extreme heat and/or dryness.

FataMorgana