Graphically, ACNH is miles ahead of ACNL, just because we're looking at updated rendering techniques and a bunch of other technical stuff I don't have the vocabulary for. Aesthetically, whether ACNH or ACNL is better is all personal preference. The big trends of ACNL were overtly fantasy-like: pink towns, pastel towns, fairytale stuff and lots of sweets and cartoon-scaled props. ACNH's biggest trends skew just a little more toward realism: suburban towns, mostly Japan-inspired, and the various iterations of "cottagecore," an idealized form of rustic minimalism that, although influenced by whimsical sources like Ghibli films and shows like "Over the Garden Wall," tends to come across as a little self-serious.
In both ACNH trends, the palettes are subdued and there is an emphasis on very small details and textures in the environment. And even though we can easily identify these different trends in both games, that doesn't mean all players are adhering to any of them! So there is no way to really say "x is better than y" or whatever. But we can (and definitely should!) compare the aesthetic trends between the games and draw conclusions about what the comparisons "say" about the players and the historical and cultural contexts that the games emerged from. Well, I like to do that anyway. It's a fun way to understand the times we live in.
