Yes, I do agree with you on this, the star sprites just look like they have too much going on, compared to other sprites.
(Found out that the Kageboshi sprite image on the wiki is from imgur, thought that was somewhat interesting.)
All of these stars basically have just one color with different shades of the same. Wind Circle has only shades of green, Ice Star has only shades of blue and a tiny bit of white, etc. The sprites that change the most are probably Doom Circle, with its somewhat drastic change from black to dark red, and Kageboshi, going from very dark purple to almost white purple.
tl;dr the stars look too different from the in-game examples, and I propose the colors mimic the style of actual stars.
Hey youâre right, half of them look nothing like the actual spells.