Is worth accepting to review manuscripts in low quality journals I usually mention the journals I'm reviewing for, as it can counts as a position of trust (i.e. to improve the quality of science).

My question is, how connected is the reputation of a journal someone is reviewing for, with that part of his/her CV. What effect would it have to the CV (and whoever reads it) adding low quality journals in the list of journals where the person is a reviewer?
Would you consider it negative, neutral or positive?

I think that if they are only bad journals, the lack of good ones might be considered a bad thing, so avoiding mentioning the journals the person is a reviewer might be preferable.

If it's 1-2 in a list of a few respectable ones, then it would be neutral or even positive, as you try to improve the not respectable journals.
I think a lot depends on what you define as "low-quality journals". If you mean lower-tier but legitimate journals (typically subscription-based, or free OA) with high acceptance rate and low impact, listing these will have neutral to slightly positive effect. Indeed, someone has to start curating the quality in order for some of these to raise in quality and impact.

If you mean fake, pay-to-publish "predatory" journals then the effect will be strongly negative. By accepting to review for these journals you are encouraging a business that hurts science and scientific publishing. Also, you're wasting your time since these journals will accept papers on the basis of whether the authors paid, not on the reviewers' recommendation.