I honestly just wish they didn't sexualize Ashi as much as they have. That's my only complaint about the whole thing. That and the romance here is pretty bland in terms of writing.
The whole 70 year old non-aging immortal coupled with the traumatized 20 year old is an odd thing to complain about because it's not something that's ever gonna happen in real life. I also object to the idea that Ashi isn't an adult capable of making her own choices because of said trauma. Especially since it's not like this show is some realistic psychological study of how something like being ageless in a time not your own would TRULY affect one's mental state. I can understand how it might be creepy in some contexts where stuff like the psychological trauma these people experience is treated seriously, but the show hasn't really done that. If it did, the resolution to Ashi's shattered worldview wouldn't be as simple as "So, Aku is actually bad and I'm actually good. Got it? Good." and Jack's return to form wouldn't be him meditating for a few hours and deciding that he had enough of being a crazy hobo and wants to be balanced again, so he tells evil jack to fuck off and that just solves all his personal problems. It's pretty lighthearted and pulpy and this romance just follows through with that.
The whole thing just comes down to the idea that Samurai Jack didn't need a love interest. I don't object to having one on principle, but I'd have preferred them doing something more unconventional than what the did. As it is, it's just a typical "Start off enemies, but the unrealistic, unfailing kindness and infinite patience of one party eventually melts the icy exterior of the other, after which it's mostly just a matter of sexual tension." It's mostly stuff I've seen before.