okay, let me give you a full bodied answer. i'm omitting jordan immediately because it's an easy answer. curren is also disqualified— she is popular, even if people miss the forest for the trees it technically doesn't mean she's underrated. so have a list!!
mejiro palmer - i actually don't have her ingame. i'm trying to avoid this kind of thing, but i want to include it since it can't be helped when i've done everything i can so far to make it happen 😭 she's a fascinating character with an interesting and well represented real life history. from her status as a sort of faux pariah among the mejiros to her lowkey onesided dependence on helios... the game seems to go much deeper into it than the anime. naturally since the anime is about teio and mcqueen.
hishi miracle - the so called "normal girl". she's actually a monster. what do you mean, 3 G1 wins? ofc number of wins=/= strength, but you understand when ypu look into it. she's a character people overlook due to erroneous memes about her being fat and lazy (🙄), and her contemporaries like gimusan, fine and kris s are much more popular, so is topro (same sire). now mirako is still holds a respectable popularity, but i really do love this every day girl who does not know how strong she really is. she self deprecates and compares herself to others constantly, but she's just as talented as they are. i love how sweet she is and how she does things at her own pace, how she makes an effort when it matters. when dantsu is released, people will see the disparity between the two of them. miradan let's go.. ☺️✨️
north flight - criminally underlooked character. i think mares tend to be, but it's even tougher if you're a miler or sprinter. of course i really enjoy the irl history, but also her rivalry with bakushin o in canon is pure HEAT. you get to see a side of bakushin o that only comes out around fuuchan, a somber side... i also love her interactions with the other tony bin kids (i'm a big tony bin fan in general). fuuchan being everyone's fashionista is also so endearing, she reminds me of someone i know irl ☺️ (personal bias, i know)
tap dance city - this is the number one most overlooked uma to me. if people knew the insane fuckshit she was on at least in eng speaking circles she would be much more popular. for one she came to japan the same day she watched ace win japan cup. zero language skills. her friends called her insane, but by the time she's in tracen she's already ~80% fluent. she has like 10 girlfriends who follow her around everywhere (not a joke or exaggeration). she wants to be rich so she can build a giant castle to house said girlfriends (+00 gen). she has no sense of reality. she hates common sense. doesn't take no for an answer. INSANE character. bonus that the trainer in her scenario had a similar personality to me so that was a bonus. i feel like if she had a more masc design she'd be in the yumejo top 100 every year, but the design we have could not be more perfect. unironic badass even though she's crazy. did i mention lesbian GOD. insane implementation of the real horse which i approve of
i wish i was joking about any of this but it's all true.

shortlist of umas i think are underrated but am not yet comfortable enough (didn't read ikusei yet, skimmed it, don't even have ingame etc): meisho doto (only skimmed and need to reread but PEOPLE ARE SLEEPING ON THIS BEAST), kawakami princess (need to read but what i know is 😶 holy shit), royce and royce (not released yet), maruzensky (don't have), super creek (i read it, but i need to reread severely it was like the second ikusei i ever read)
i think 93 gen is underrated in general.