Is apparent horizon curvature lesser due to refraction of light in the atmosphere?

Yes -- because refraction influences the apparent distance to the horizon, it also has an effect on the curvature.

To visualize this, it might help to think in extreme cases, for example in the case where due to refraction the horizon is at an apparent distance of only 1 meter. In this case, the curvature of the horizon would be extreme (it would be a circle of radius 1 meter around you).

In reality the curvature effect is much smaller of course, and I doubt that it is visible.

(As an aside: one result of atmospheric refraction that is observable is a phenomena called 'the green flash'. Because refraction is colour-dependent, the red and yellow part of the sun could already have set, while a small part of the 'green sun' is still above the horizon. This can be observed by the naked eye, preferably when the horizon is sharp.)