I think Birthday candles are great for spells but like any other type of candle, they have their pros and cons.
On one hand, we all grew up making wishes around them so they are perfect for a simple wish spell (they burn fast). Also, they come in many different colors and are very affordable.
On the other hand, it’s hard to carve on them or decorate them, and they can distract you from the ritual you’re doing. You’ll probably have a harder time setting the mood when it looks like you’re about to blow a birthday cake!