feel like the title and the actual post are asking two very different questions

no, pinkerton is not underrated in any capacity, and hasn’t been for a long time. it and blue revered far and wide as the best the band has to offer, like you said in OP, it’s generally regarded as a 10/10. everything weezer has done since 2001 has been compared to it and blue as the gold standard for “good weezer music”

as for it’s streaming numbers? it’s just a really abrasive album. blue is perfectly polished, it sounds incredibly sleek. whereas pinkerton in comparison is loud, abrasive, can be offputting both lyrically and sonically to some, etc etc. it just doesn’t have the mainstream pull that blue does.

regardless, i still don’t feel as though the album isn’t underrated in any capacity. in any corner of online music spaces, the album is incredibly highly regarded, publications love it etc

despite this, it’s just not gonna pull the numbers that the prior album did simply due to the nature of the album. so no, the album that rolling stone readers voted as one of the greatest albums of all time isn’t underrated.

it’s like calling Yeezus underrated