Is 100 degrees an obtuse angle?

An obtuse angle is any angle greater than 90 degrees and less than 180 degrees.
Yes. An obtuse angle is geometrically greater that a right angle. Any number degree larger than 90 degrees is obtuse, as any number degree lesser than 90 degrees is acute.

An easy way to remember this:

Acute - A cute lil' angle

Obtuse - Obese, big angle