Does litotes need to be negative?

Neither of your examples are litotes, but let me answer your main question: Does litotes need to be negative? It depends on how strictly you want to define the word. According to some sources, no, it doesn't need to be, although a negative construction is the most common form.

Some sources do define a litotes as requiring an overt negative. One example is ODO:

Ironical understatement in which an affirmative is expressed by the negative of its contrary (e.g. I shan’t be sorry for I shall be glad).

From a quick review of other sources, it would seem that this strict "traditional" definition has given way to a looser approach where negation appears to be optional. Dictionary.com:

noun, plural litotes. Rhetoric.
1. understatement, especially that in which an affirmative is expressed by the negative of its contrary, as in “not bad at all.”.

Wikipedia gives a more comprehensive summary, including the following:

Litotes is a form of understatement, always deliberate and with the intention of emphasis. However, the interpretation of negation may depend on context, including cultural context.

It seems reasonable, therefore, to say that litotes always requires both understatement and negation, but the negation may be implicit rather than overt. Appropriately, The Forest of Rhetoric (thanks @Icy for the link) offers two examples of litotes, one of which lacks the overt negation-of-opposite element:

It isn't very serious. I have this tiny little tumor on the brain.
—J.D. Salinger, The Catcher in the Rye