Poplar is pretty soft. It will be easy to work, but will not hold up to abuse as well. The maple usually sold at the big box stores is soft maple, but is harder than poplar. If you can find a piece of hard maple, it will be harder to work, but will hold up the longest. That has been used for bowling pins. It is also a diffuse-porous wood, less likely to hold wort and grow bacteria than oak or ash, which are ring-porous and, well, more porous. Wood finishers will use fillers to fill the open grain on those, but I don't know how those fillers would do in 150* wort. Bacterial growth shouldn't be a big deal for an hour mash pre-boil, but I'd still make sure to rinse that pretty ash paddle well fairly promptly after use.