i guess someone going from the movie--where howl is presented as a romantic ideal--could assume that book!howl was intended as the same. but he very clearly isn’t--not only is he shown to be obnoxious and bad at relationships, he and sophie (and his sister, and mrs pentstemmon, and the people of market chipping...) explicitly point out his flaws at various points, so for the reader to draw attention to the red flags feels like... yeah duh?
a central part of the final scene is sophie & howl choosing a happily-ever-after together that will be “hair raising” where she says he’ll exploit her and he says she’ll cut up his suits to teach him. if you read that and think “that sounds like a terrible relationship, i wouldn’t want to be part of it” that’s a valid reaction, but you will probably also be perplexed by the book’s broader appeal
when i first read hmc i was caught off guard by the sheer catharsis i felt when sophie and howl choose their weird, hair raising, maybe even unenviable relationship, because the message it conveyed to me wasn’t “this is what a relationship should look like” but “you can find someone who is delighted and endeared by the things everyone else told you were failings.” most of howl’s flaws are comically human: lazy, vain, spends too long in the bathroom, scared of getting his ass beat by an angry ex, magnified into a larger-than-life fairy tale character, and like @ifitisnotayesitisprobablyano said, sophie is an equally flawed character, in equally human ways
and the book is fundamentally about sophie growing into herself, in a way that involves becoming confident and unapologetic and unpalatable and happy, and so a romantic subplot that involves finding a normal, palatable, ideal partner--or worse, finding howl in all his imperfections and “fixing” him--would definitely change the themes communicated by the story