You might be able to compost clumps of grass – but it’s a bit more problematic than you might think.
Grass clippings are generally a great addition to a compost heap but clumps of longer grass often still have some roots attached so may be inclined to regrow (or keep growing) rather than rot down. You can reduce the chances of that happening by shaking off soil from the roots and leaving the clumps to dry out before adding them to your compost bin.
Clumps of grass are also unlikely to be a grass monoculture – unless you feverishly attend your lawn like it’s the 18th hole at St Andrews or the pitch at Lords, it’s likely it’ll contain a few different types of grass (possibly including couch grass) as well as “weeds” (buttercups, sorrel, clover, yarrow…) and mosses. The seeds of those grasses or weeds may not get broken down in the composting cycle and will regrow when you come to use the compost, if not actually in the heap itself.
That said, if you have a fairly efficient compost heap, you’ll probably get away with it, especially if you mix them into the hot bit in the middle – they’ll break down quicker and the lack of light will discourage the plants from regrowing there.