No, modding isn't against blizzards EULA (at least not with starcraft)
their legal faq mentions modding and says they allow it, so long as the mods follow the EULA's rules
heres most of their legal faq that talks about mods:
QUOTE(Battle.net legal faq)
Can I make add-ons or expansions for Blizzard Entertainment® games? Can I sell them?
We do not allow add-ons or expansions for Diablo™. You may make and distribute StarCraft™ and Warcraft® maps and campaigns that you have created yourself, so long as it is for personal, non-commercial purposes. Any such maps or campaigns would also be subject to the other terms outlined in our End User License Agreements included with those products.
QUOTE(Battle.net legal faq)
Can I sell or charge for a CD or other media containing maps, add-ons or campaigns found on the Internet?
No.
QUOTE(Battle.net legal Faq)
Can I get authorization to do an expansion pack, novel, screenplay, theatrical production, or other adaptation based on your games?
Unfortunately, Blizzard Entertainment® cannot accept unsolicited ideas or submissions. As a result, we will not be able to review or authorize any submissions related to expansions, novels, screenplays, productions or other adaptations.
QUOTE(Battle.net legal faqs)
Can I do a total conversion of your games?
Yes. We've seen some very polished and fun conversions for our games, and have no problems with total conversions so long as they are for personal use and do not infringe our End User License Agreement included in our games, nor the rights of any other parties including copyrights, trademarks or other rights.