I would use extreme caution on Groening sketches. It's a very simple autograph that could easily be convincingly replicated.
This example in particular seems to be carefully and deliberately applied. It may be authentic, but it could also be a halfway decent trace job. I don't know.
My suggestion would be to weigh all factors: autograph analysis, provenance, etc before making a purchase.
Thank you all for the POVs. As noted, Groening SIG and sketches are fairly basic so expert knowledge on what to look for is invaluable. Disappointing to hear concerns but this is exactly why I joined the forum. Appreciate the feedback and education.