How to interpret Sum of squared error
SSE in classification is proportional to the Brier score:

$$
SSE = \sum_n \sum_k (\hat y_{nk} - y_{nk})^2 \\
Brier = \frac{SSE}{N}
$$

For observations indexed by $n$ and classes indexed by $k$.

In your case, you obtained a Brier score of 0.18.

The Brier score can be decomposed into

$$
Brier = Reliability - Resolution + Uncertainty
$$