hearing loss that is much worse in the high freqencies than in the lower frequencies is referred to as a "ski slope" loss.
The name comes from the shape of your hearing ability as reflected on the audiogram. That line starts high and then falls off as the frequency increases.
Ski-slope losses are, by far, the most common type of hearing loss. The result of a ski slope loss is that you will have trouble hearing the sounds that distinguish consonants, and that will make it sound like people are mumbling.
Although not as common, other types of hearing loss exist and are commonly referred to by the shape they have on an audiogram:
Older hearing aid technology could do a pretty good job of helping someone with a moderate ski slope loss, but didn't do so well for more severe ski slope losses. This led to the misconception that persists to this day, that "hearing aids won't help". You may even get this incorrect story from professionals who should (by now) know better. Modern hearing aids are much more capable of helping even people with severe ski slope losses.