No. If the higher rated player accepted the challenge and the game is NOT aborted by the server, then it is rude of the player, not bad sportsmanship.
Sportsmanship
If a higher-rated player accepts a challenge from a lower-rated player, should the higher-rated player abort before the game begins?
They may be ignorant of the ways of the ratings filter.
If a live game send this link, highlighting the "Tips to Avoid Aborting Games!" section. http://support.chess.com/Knowledgebase/Article/View/171/0/fair-play-policy
If an online game send this link http://www.chess.com/home/echess_settings.html
I say it is never impolite to decline a challenge. EVER. You don't have to play anyone, no explanation needed, but you should be polite about it.
Conversely, it is ALWAYS impolite to ignore a challenge. Declining is the only option if you don't want to show that you are a smacked a$$.
And UniqueAirplane'ss post is high irony since he (apparently) pretends to be concerned with protocol, yet ignores my challenge instead of declining it.

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If a higher-rated player accepts a challenge from a lower-rated player, should the higher-rated player abort before the game begins?