De-meatifying lasagna, casseroles
Meat lasagna minus meat will not be as satisfying - it'll be missing both a flavor and texture component. And tofu by itself does not have much flavor, especially compared to the rest of the lasagna.

Substituting for meat is a broad question, as you'd see if you'd tried searching for vegetarian lasagna. If you're determined to make something that's as close to lasagna with meat as possible, then you probably want to use something that's actually intended to be a ground meat substitute. There are various options, but they're generally soy- and gluten-based, with some sort of flavor. This will be good for vegetarians who wish they were eating meat.

But for vegetarians who don't feel the need to pretend they're eating meat, and especially for ones who don't want to feel like they're eating meat, you're asking the wrong question. Don't ask what to replace the meat with. Ask how to make a good vegetarian lasagna-like dish. The flavor of mushrooms works well (including plenty of umami - note that mixing varieties gives the broadest flavor) and provide some nice contrasting texture to the rest of the dish. Peppers and onions also have great flavors, and don't drift too far from the original flavor. I tend to enjoy spinach and eggplant too. Just try searching online for vegetarian (or vegetable) lasagna, and you'll find tons of variations. They're not all for everyone, for various reasons, but there's plenty to look through for something that works for you.

As for tofu... there's already a ton of cheese in a lasagna, which does have protein. Unless you're really worried about protein intake, I don't see focusing on protein as the route to a pleasing dish here.

(And this advice is pretty much all general; it applies to most flavorful casseroles that might normally contain ground meat.)