I loved Cotm when it came out, still think it is a solid entry. However, the scores it was given by gaming sites at the time puzzle me. IGN for example scored it higher than Sotn, which I don't really understand. I have got much love for it but it's overrated to me.
Review scores rarely reflect actual quality, so there's that.
COTM is a rare highlight of the series in my opinion. It doesn't set its expectations too high, but looks at the errors of the Gameboy Castlevanias and says "let's avoid those" and then looked at Symphony and said "what can we realistically include from this with the hardware we have?".
Remember that the Gameboy Castlevanias were finicky, odd to control, and didn't really have a great feel to them. Of the three, Belmont's Revenge is probably the best, but that's like saying that canned spinach is better than moldy spinach and overcooked canned spinach. Of course it's better, but that doesn't make it something you want to have all the time, and you'll easily spring for something else almost as soon as it becomes available.
Circle of the Moon, as the first GBA Castlevania, was effectively charged with redeeming the series portable legacy single-handedly, and I think it managed to do exactly that very appreciably.
It also gave IGA a lot to go on when he made Harmony and Aria, and neither game would have been as good had COTM not come along first to show how a handheld Castlevania could be done
well.
In essence, just as SOTN redefined the series on consoles, COTM redefined the series on handhelds.
In that sense, it's not overrated at all. If anything, it's
underrated as many people forget it even happened today.