According to the Hanafi school of thought, situations that invalidate ablution include the discharge of blood, pus, or similar fluids from the body. However, for these discharges to break the ablution, they must be in a significant amount.
A mosquito bite does not break the ablution. However, if the bite causes bleeding or any other condition, it may affect the validity of the ablution. In cases where there is no such issue, a mosquito bite does not invalidate the ablution.