It's just the stiffness does not sit well with me.
This sentence tells me you don't actually know anything about Foster's animation. Far from stiff, in fact better to say loose and wobbly. So I can prove an educated answer rather than repeat facts from other researchers, as you guys are doing, here are some Foster scenes at Terrytoons:
Foster does most of the scenes with the eccentric, loose, rubbery dog chaser in THE DOG SHOW (go to 3:50 and 5:00)
An animator that is "the worst" and "stiff" wouldn't of been able to animate the loose, nicely timed, funny scenes depicted here. The little touch of Dr Moan's nose moving when looking into the Farmer's mouth isn't something a crude animator would've done, it takes consideration to do that
Finally, Foster handles the scenes in the last minute of FLYING OIL where the salesman tries to sell the Farmer into buying his product, only for both of them to shoot up by cats (these scenes handled by other animators). Foster does nice touches with the streak lines on the salesman's finger when he twirls it, some expressive posing, and nicely drawn effects when he launches them (followed up by Frank Moser's work). Again, a "worst" animator certainly wouldn't of been capable of this
With this in mind, what do you guys consider a good animator in the first place? Hopefully this has at least given you consideration into more than a basic, poorly formed opinion on Foster. I hope it wasn't a waste of time for me to write this, I truly do like his work