It's funny and surprising hearing how rare they are. Where I live (British Columbia), they're extremely common (although a shot from an interesting perspective such as yours would take a bit longer to find + a longer lens).
Since it was on the spot, you've got whatever lens you've got. What I would change is your ISO, aperture, and shutter speed. The rule of 1/focal length for sharpness doesn't entirely apply if you're cropping in (effectively using a longer lens but using less pixels), or snapping the photo in a frantic hurry. I would've tried to take this shot at 1/200 or a little quicker even. Let's just say you needed 1/250 for a really sharp shot. I also would've taken it at f2.8, maybe f4 (to maintain some desirable bokeh, but to increase your field of view & get better sharpness for cropping, and to increase your chances of nailing the focus). If you shot this at 1/250, f2.8, and ISO 400, you would've ended up with a much sharper image. Another possibility is 1/250, f4, and ISO 800. Don't be afraid to bump your ISO up, but do remember there's trade-offs with any setting: If you need to crop in heavily, the noise from higher ISO will become even more apparent (but noise is usually better than unwanted blur).
I do like the shot. The composition is nice, the whole squirrel is in the shot, and the angle of the shot is interesting. Do note the f1.8 focus though: You got the tree slightly in better focus than the squirrel. So the two issues are camera shake (likely due to a slower shutter but hand-holding technique can help lots), and slightly missed focus due to such a shallow depth of field.