Some of my trees are producing a lot of flower buds while they should be producing new leaves instead (some are in bad shape after a red spider mite infestation). Is it wise to pick off the flowers so that the tree has more resources for leaves??

Flowers don't matter much, but if you want to maximze growth, you should remove any fruit that sets.

As Skeeter points out above, the removal of flowers, has very little direct affect aiding additional growth. Further, according to The American Phytopathological Society's scholarly publication "Citrus Health Management", the elimination of fruit from young citrus trees, also does not promote additional vegetative growth under many situations. The long held belief, that removing fruit from young trees promotes increased vegetative growth, in most cases, cannot be justified as always being correct. Here is a direct quote from the APS publication ...."Trees may produce a little fruit during the first two years in the grove. This fruit is removed in some cases, so it will not compete with and limit tree growth. This growth inhibition occurs ONLY with VERY HEAVY crops, however, and the removal of fruit from young trees prior to commercial production is seldom justified". - Millet (1,337-)