The loss of ones colon does not significantly alter the immune response. The presence of an inflammatory disease can have a larger effect, though, even without a colon.
The story is more complicated than a “strong” or “weakened” immune system. For one thing, the immune system is a bunch of separate systems working together. For another, the worst consequences of some infections (like Coronavirus) seem to be from an overly aggressive immune response, rather than from a failure to recognize and fight the infection.
I’m not aware of any data about the effect of underlying IBD on Coronavirus.