Tender and stiff are common terms used to denote initial stability or "GM". They do not apply to ultimate stability nor to range of stability. Much depends on how the vessel in question derives her stability. Is it from form stability or is it from a well ballasted design? A boat with large initial stability derived from form stability may prove to be inadequate in heavy weather while a vessel with large weight stability may be initially more tender yet offer a greater range of stability.
GM, or the height of the metacenter above the center of gravity, is a measurement of initial stability and will be reflected in the "tippiness" cited above. But it is a small and potentially misleading portion of the overall stability picture. For a more complete picture one must look to righting arm and righting moment at various angles of heel. A relatively flat bottomed fin keel boat may have the same GM as her long keeled 'V'-shaped hull brethren yet her righting arm, GZ, and righting moment, RM, will likely be substantially less.
Little can be determined by just stepping on board and finding a boat to be initially stiff. And it is also quite possible for a boat to be excessively stiff, which can come into play in heavy weather sailing or in a hove-to condition. Much depends upon the intended usage of the boat as to what trade-offs in stability and hull form design can be labeled as desirable.
There are numerous books on the matter of yacht design as it relates to intended usage. Two good ones on the subject are C.A. Marchaj's Seaworthiness: The forgotten factor and Lars Larsson and Rolf Eliasson's Principles of Yacht Design.