depends on how one defines "entertainment". technically everything could be a form of entertainment.
entertainment: 1: the act of entertaining 2: Maintenannce, provision 3:something diverting or engaging a: a public performace b: usu. light comic or adventure novel
one could argue that there is some art that is not diverting or engaging but still qualifies as art. so.....no, art is not a form of entertainment if you are using the Meriam Webster Collegiate Dictionary definition.
but if you change it to 3: something that ATTEMPTS to divert or engage the criteria is switched to intent which can't be really be proven or at least is dependant on whether or not the artist states that is the intent and whether you believe the artist or not......so some art can qualify as art but be done without the stated intent to divert or engage. so no, by that definition it is not a form of entertainment UNLESS definitions of "entertainment" are proven to be completely useless.
~M.E.