If by spotlights you mean the small round recessed halogen downlighters fitted into the ceiling, then provided they are suitable for use in a bathroom, pretty much any type could be dimmed.
Make sure the dimmer is a decent one, and can actually be used with halogen lamps.
If the lamps are 12V types, also ensure that the dimmer and transformers are compatible.
However dimming halogen lamps is a bad idea since it will shorten their lifespan significantly.
Halogen downlighters are a grossly inefficient way to provide lighting.
What would be the point of dimmable lights in a bathroom anyway?
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