It's hard to be nice to 'Tragedy Day'. There are two basic tacks you can take when writing about it; you can either say that it proves that Gareth Roberts was really better suited to write something other than the Seventh Doctor, or you can say that it was a work of juvenalia that sorted through ideas he later did a much better job of handling in his other work. Let's do both, shall we?
First, let's talk about Gareth and the Seventh Doctor. It's become more and more obvious over the years that Gareth Roberts is a warm, fluffy, huggable teddy bear of a writer who loves writing gloriously silly romps. He is a champion of the Graham Williams era, and has done an excellent job of pastiching it and capturing its humor, wit and charm. He's written for Tennant and Smith and Eccleston, and every one of his scripts is filled with glee and laughter. His Big Finish audios are comedy classics. In short, Gareth Roberts = fun and frolic and froth.
But the New Adventures were never particularly frothy, even after 'No Future' when they finally reined in the apocalyptic dysfunctionality of the TARDIS crew to manageable levels. Ace remains a hardened soldier and full-tilt badass, the Seventh Doctor is still a manipulative bastard (and I'm suddenly picturing the Tarantino Doctor Who story, "Manepulativ Basterd") and the Whoniverse is Grown Up and Serious. Trying to do a comedy in this line of books is like swimming the English Channel dragging an anchor. I'm not saying it can't be done, but it's difficult enough to make you wonder why anyone bothered.
In 'Tragedy Day', you can feel the anchor a lot. There are at least three too many gruesome deaths of innocent people to really enjoy the comedy surrounding it, and even the death of the main villain feels awkward and unpleasant because (spoilers) Roberts came up with the bright idea of making the villain a twelve-year-old kid. Which yes, funny that a kid is behind everything, but less funny that a kid gets crushed by girders. There's a constant, unpleasant dissonance in tone that makes the work feel like a Frankenbook, comedy and horror stitched so badly together that you can see the joins.
And, to shift to the second tack, the comedy isn't all that funny. Everything Roberts does here, he does better in 'The One Doctor'. A villain with a scheme to turn a whole planet into a 50s sitcom is a vaguely amusing juxtaposition of adventure-story tropes and mundane domesticity, but it doesn't do the job nearly as well as forcing the companion to assemble dimensionally-transcendent shelving to placate psychotic furniture-packaging robots, while the Doctor is on a Quiz Show of Doom. Roberts got the hang of this as he went along ('The Lodger' is another good example of him hitting that sweet spot between "normal life" and "madman with a box") but 'Tragedy Day' goes on too long and doesn't have enough jokes to make it work, even if it didn't also have the minor problem of being bleak and miserable. (Although some of that may be ironic; the Friars of Pangloss are so over-the-top EEEEEEVIL! that it almost does become funny through the back door. But there's too much going on to be able to make that stick.)
Basically, the nicest thing you can say about 'Tragedy Day' is, "He gets better." And he really really does...so let's just look at this one as an early work and move on.