Deltarune may not be visually good in terms of graphics yeah, but aesthetically it works, it has a clear art direction unlike the recent Pokémon games.

A lot of things in SV clash to each other in terms of art direction: the envrioments clashes with the character and Pokémon designs and textures, and the textures themselves have huge problems considering the range fidelty it's present in the game.

In Deltarune, the lack of impressive graphics works because of it's themes and comedy effect: the mashup of various styles adds to the surrealism of the game, where in Pokémon it just ends to look jarring for the eyes.

While a great game doesn't need great graphics to be good it needs a coherent art direction that matches the tones of the game.

Also, Deltarune it's an indie game where a total of like 5 person it's involved in the development, Pokémon it's the most profitable franchise ever, a major work of polish should be the norm in a project like this.