If a defendant's counsel does not raise a mitigation argument, is that considered ineffective assistance of counsel?
Sometimes it is, sometimes it isn't. The test comes from Strickland v. Washington 466 U.S. 668 (1984), reiterated in Woodford v. Visciotti 537 U.S. 19 (2002):
[A] defendant need only establish a "reasonable probability" that, but for counsel's unprofessional errors, the result of his sentencing proceeding would have been different.
Woodford held that failure to present mitigating evidence did not prejudice this particular defendant's sentence.
There is no support for the conclusion that the state court failed to take into account the totality of the available mitigating evidence and to consider the prejudicial impact of counsel's actions. The state court found that, because the aggravating factors were so severe, respondent suffered no prejudice from trial counsel's (assumed) inadequacy.
Schriro v. Landrigan 550 U.S. 465 (2007) found that counsel was not required to present mitigating evidence after defendant interrupted him, telling the court to "bring on" the death penalty.
Rompilla v. Beard 545 U.S. 374 (2005) held that:
[e]ven when a capital defendant and his family members have suggested that no mitigating evidence is available, his lawyer is bound to make reasonable efforts to obtain and review material that counsel knows the prosecution will probably rely on as evidence of aggravation at the trial’s sentencing phase.