mbstring is used to deal with MULTI BYTE characters, meaning that it handles euros, and languages with multibyte characters (such as Russian, Chinese, etc) properly, instead of splitting an 8 byte character in to 4 bytes, then lowering whatever the 4 byte char is (which is normally not the equivalent of the 8byte character).
If all your forums will ever process are English words, and no special characters or emoji's (ðŸ˜‹ðŸ˜ðŸ˜ŽðŸ˜ŠðŸ˜‰) then you should be fine, and SEO urls will not break. Multibyte characters can also be in page descriptions.
Though mbstring is normally included in the default install of PHP 5, its seemingly off when a host upgrades from php4 and turns off every feature from php4 along during the update to php5.
There are numerous discussions here regarding some hosts not supporting mb_convert_case, usually its as simple as sending an email requesting it be enabled on the server; though you're on GoDaddy and that seems to be like talking to a brick wall (feel free to use the search feature
TLDR; you shouldn't have to disable this bit of code, your webhost should support mbstring as a basic part of its PHP5 instance.