when a regular (non-expert) witness does not like the defendant (and admits it when asked), does that alone give a valid reason to discredit them to any extent at all?
NoNot alone. It is simply another factor the jury may consider in weighing a witness's credibility. U.S. v. Bonilla-Guizar, 729 F.3d 1179, 1186 (2013).
If anything, from a practical standpoint, the witness's disclosure of bias suggests that his testimony might be more reliable insofar as the witness is prioritizing truthful answers (even those that at the outset might prejudice the testimony altogether) rather than credibility calculations. A transparent disclosure of [dis-]liking the defendant gives the jury a straight-forward element on how much credibility the testimony deserves. The court in Bonilla-Guizar points out that the witness's admission of bias "may have had the effect of enhancing his credibility".
Embry v. State, 923 N.E.2d 1 (2010) addresses evidentiary issues of which the premise is the witness's exposed bias against the defendant. For instance, the opinion points out that "[in] some jurisdictions [...] the State may respond by introducing the defendant's prior uncharged misconduct to explain the witness's antipathy", Id at 7-8. The Embry court took the opposite approach only as to the prior uncharged misconduct, which is short singlehandedly dismissing the witness's testimony.
See also State v. Wacaser, 794 S.W.2d 190, 196 (1990) ("A prosecutor is not necessarily ineligible because he does not like the defendant.").