Two years even further on tough, Lobo had already started to wear out his welcome... part of it was that he worked best as a mature readers action comedy, part of it was he was fairly complicated to draw well (Simon Bisley was the only one to truly draw Lobo with any biker/punk authenticity). The biggest part of it was that he's a fairly one-note character and only really good for providing heaps of violence. He was the Deadpool of the 90's.
For some reason, in 1994, Lobo appearing in a Superman book was considered an event... or at least event enough to merit a gimmick cover. Of course at this point in time, the gimmick cover was as prolific as sales booster as "guest appearances" by hot characters, and a publisher would use any excuse to trot out another one and charge a few bucks more in doing so. Plus, the speculator boom was in full swing so a ginmick cover would stand out to them. Combined with a "hot character" appearance, it was a theoretical cash bonanza.
Gimmick covers these days are, by wary fans of old, considered a waste of money. Put the money in good story and art. But given that in the 1990's the material in the books was so mediocre (if not outright terrible) that the gimmick cover was at least something interesting for the buyer to look at (or sometimes play with). Man of Steel #30 was not, and still is not a good story... it's a thoroughly inorganic meeting between Supes and Lobo that leads to a pointless fight and an unlikely truce at the end. Bleh. But the cover... that's fun.
Touted as the first ever do-it-yourself cover, this special polybagged issue came with a couple dozen colorforms of Superman and Lobo in different poses and some other scenery bits. The reusable vinyl sticker-ish shapes applied nicely 19 years ago but they aren't so nicely reusable now. I wonder had I kept them on the sticker sheet instead of cramming them all onto the cover, would they be any better?
As far as I know, the only other use of this gimmick was a few months later in the one-shot finale to the "Worlds Collide" crossover between the Superman (well, Man of Steel, Superboy and Steel) with the Milestone universe (I don't think I took the appliques off the sheet on that one so I'll have to dig it out and see if they still work at all).