I tend to agree with travis82, lxtask is minimal but functional .. if users want something heavier (even if only a bit) they can install whatever they choose.
The Peppermint ethos dictates we don't make software choices for our users beyond what is necessary
.. in fact I'd be more inclined to do away with a task manager altogether if I didn't consider it pretty necessary for the vast majority.
A few kilobytes may not sound like much, but if we did that with ALL packages.....
That said, show me a function of another task manager that the vast majority of users would consider "necessary" and I'll certainly consider it .. but you'll have to tell me the exact name of the package, "system monitor" doesn't tell me much