It's not that. HotS is generally more zoomed in/has fewer but bigger assets, which demands more detail. SC2 on the other hand must allow hundreds of units at the same time without performance breakdowns. I also think HotS is a bit too bright for SC2. I remember how people complained about the bright style when SC2 was first announced.
Haven't played HotS in a long time (last new hero I remember was Greymane - January 2016)), but I'm sure I'll try out the new map.
Dear Gradius... The last assets for sc2 lotv (i am not talking about primal zerg) doesn't seem at all cartoony, especially on nova covert ops, i like more and more the model for sc2, i mean... the doodad and units model from lotv and after are amazing