Yes, abnormal sperm is able to fertilize but most likely will not since those with normal shape move faster and will have it easier to reach the egg.
Depending on how the sperm are altered and how many abnormal sperm there are, the likelihood of pregnancy will be higher or lower.
For example, in spermatozoa without head (no genetic content) or with a small or absent acrosome (necessary to penetrate the egg) fertilization will not take place since they lack an essential part for the process.
Suggested for you: What Is Sperm Morphology? – Meaning & Normal Range.