I have developed couple of hats having own micros. In the prototypes I was taking power from the 3.3V RPi. I know that it is not recommended, so I have added own LDO connected to the 5V pin. Strange things started to happen (random resets of my micro). I had to add 10u electrolytic capacitor on the LDO input (there is another 1u as DS advises) to stop this to happen. Is the voltage on 5V pin as bad as it looks like? Maybe it is only my RPi.
"I know that it is not recommended" - why do you think this?
The 3.3V pin is there for PRECISELY this reason, and can supply up to 800mA.
The Pi actually has an on board regulator which will perform better (and more efficiently) than any linear regulator you can add.