No, is amazing what you can do with the humble 8 bit 16MHz Arduino. It's just a matter of knowing how to get the best out of what you have.
You say dimming, but do you mean overall dimming for all LEDs, or individual dimming for each led independently? By the first of those two, I mean you can control the on/off state of each led individually, but the dimming level of all the on LEDs is the same.
It can be done with shift registers if you use SPI hardware built into the Arduino, code efficiently, use bit-angle-modulation etc.
A ht16k33 chip can drive 128 LEDs individually and provide overall dimming. 2 X max7219 also.
Tell us more about your project and we can give more specific advice.