Overrated
Mariah Carey- she was...something in the early-mid 90's but then her voice got really tense and noisy. People also do not realize how often she either used full studio playback, prerecorded versions or lip synced hardest parts of songs f.e almost all "We belong together" climaxes were lip synced. She is at the same time so horrible and so good at lip syncing that sometimes u can't tell the difference and sometimes it looks like she couldn't give a flying fuck.
Christina Aguilera- named "the voice of generation" and praised to be a legendary singer with 4 octave range. In reality the quality of her voice is dubious at best- airy pushed low notes, strained basically screamed belts, head voice and falsetto are the best part of ber range and she demonstrated that she can hit decent G6 many times (What a girl wants). Her usable live range is C3-G6 which is less than 4 octaves but still decent. However most of her live performances is using 100 generic runs, glissandos, repeating "hieeeeeeeeyyyy" "yeeahhhhhhjejeeje" and dodging most important high notes - Fighter E5, Beautiful F#5, the voice within F#5. She rarely use playback- most of the times its too hit hard note -like F#5 from aint no other man or long E5 from Candyman. She is def worse than Ariana or Demi a nd the latter praise her like a god.
Underrated
Lady Gaga- despite the fact that Gaga was good singer since 2008 and her abilities only became better she was mostly remembered as attention seeker with weird designs and being a gay icon. Most people do not think about her as a great vocalist. She has great vocal technique, her range is consistent from C#3/D3 up to A5. Her belting really good and resonant.
Beyonce- another one who is imagined as a dancer rather than vocalist. Her entire vocal range a d technique is nearly perfect- she supports down to B2 and can belt as high as G5 with resonance and her head voice is also one of the best in the industry. I think her and Gaga underrated perception of vocals is due to the fact that they are performers- they have dancers around, walk half naked or do other shits while Mariah or Whitney were just standing and singing. (Both are terrible dancers tho).