。
My sense of smell is dull.
Literal
Sense-of-smell [subject-が] become-dull-[state-ている].
A short statement of declining sensory acuity. 鈍る (にぶる) 'to become dull, deteriorate' is the verbal form of 鈍い ('dull, blunt, slow'). The ~ている here marks the resulting state — 'has become dull and is now dull'. The verbal form lets the speaker frame it as a process that has settled into a current condition, rather than just a static attribute.