It was very bad, at least in its final years. I went once shortly before it closed- it was by then the most shabby and rundown Zoo I had ever visited- anywhere.
The strange thing was the Director was a highly educated and respected Zoo man, but funding and upkeep of the Zoo was obviously beyond his means entirely.
This is a real can of worms - and there are some occasional posters on Zoochat who are much more knowledgeable than me, and highly vociferous, and will, I am sure, delight in rummaging around in that particular can given half a chance!
I think it would be unfair to say that Glasgow Zoo was 'very bad' - although there is no doubt that it had very bad elements. Its carnivore keeping was innovative and successful, with some excellent work with small cats in particular. Its Asian black bear husbandry was ground-breaking, at the time. Its reptile collection was good, and well done (without being massively exciting). There was quite a decent tiger set-up.
Against all of this, though, was a perennial struggle for money which showed itself in cheaply (and badly) put together enclosures, in corner-cutting management (I have heard - from a reliable source, but one with a particular axe to grind - that a camel died from malnutrition at one point), and in really crass ventures such as car boot sales in the zoo grounds.
The director you mention, Richard O'Grady, was, by some analyses, the root of the problem. This is perhaps unfair - whether a combination of Durrell, Bartlett, Hagenbeck and Grzimek could have made anything of the hand that O'Grady was dealt is debatable. But I think more could have been made of the zoo. The site was brilliant - even if it was in a less-than-lovely area of Glasgow. Glasgow is a city that really feels a sense of pride and identity. There are a large number of wealthy individuals in the city. Things might have worked.
On my last visit to the zoo, O'Grady literally tried to throw me out of the zoo (he grabbed hold of me and tried to manhandle me out of the gate). At other times, he could be quite charming. He was certainly not a straightforward fellow.