I think lying about it after the fact definitely makes the ex-copper's error in judgement worse. We need to be able to trust police officers, and an incident like this shows he wasn't a trustworthy person.
That said I do have some sympathy for him. I'm sure basically every single person in the country has done something untrustworthy at some point - be that a 'white lie' to a friend, not owning up to a mistake at work, cheating on a partner, speeding in a car when you don't think you'll get caught, etc. Some people will do these things more often than others and some will be sorry about them while others won't. The guy in question was to some extent just unlucky to get caught imo. On the other hand it was basically shoplifting which in this context is more than just a minor incidence of untrustworthiness (quite a black and white right/wrong situation with a concrete impact, so much less room for argument than with more personal examples of untrustworthiness), and with the attempt to lie about it afterwards I do think it betrays his character...
If only such a stringent attitude was taken with other misdemeanours I wonder if we'd have fewer police officers in the force who feel happy doing things like threatening to plant evidence, beating people unnecessarily, or worse... I'm not anti-police at all and by and large I'm sure police officers are good people doing a difficult job, but there are undeniably some people in the force who shouldn't be.