Is there anyone else in the entire cast who's aro and/or ace?
Krobus is definitely AroAce, as is Papyrus.
I don't know why but I always got the vibe that Maru might be asexual. Just seems so smitten with technology and robots instead. (But in SDV she does have kids with the Farmer if you happen to marry her so maybe I'm reading too much into it.) I might just head-canon that she is honestly.
Emily always struck me as aro because up until the 1.1 update you couldn't marry/romance her and she always seemed uninterested in local romance. I'd like to think maybe she still is as there is frankly an appalling lack of LGBTQ+ representation in SDV. (While Krobus is great and I platonically love my best buddy, I don't think he counts because he's a monster and falls into that unfortunate trend of LGBTQ+ characters being only non-human in media.)
I might need to think a little more on Undertale characters but I'm pretty sure MadMewMew is AroAce as well. She's got a business to ambitiously run so I don't see them doing much in the way of romance or relationships.