The P-38's weight followed its heavy-fighter mission

Did the P-38's twin engines and extra weight make it a poor fighter?

No; it was a capable long-range heavy fighter whose weight followed from the role, especially in the Pacific.

The P-38's weight is assessed against its role as a long-range heavy fighter. Twin engines and the twin-boom layout do not, by themselves, make it an unsuccessful design. The comparison with the Bf 110 explicitly rejects treating the two aircraft as having the same problems. For the Pacific missions being considered, the judgment is that few contemporary alternatives would have done the job better.