There is no requirement to keep the powered by mojoportal. The only thing I don't like to see is when someone uses mojoPortal and then puts "powered by something else".
Designer links are a separate issue because designs are under their own licences by the designers so you have to work it out with the designer if you want to remove the design by link, or else make a custom skin of your own.
Licensing of mojoPortal mainly covers distribution of software, there are no clauses about how you use it, so if you are not packaging/distributing downloads then there is not much chance of license violation. However it is allowed to package and distribute mojoPortal along with custom features (which can be under other licenses), but you cannot change the license of mojoPortal and would have to indicate that the package does contain mojoPortal licensed under the CPL. Distributing mojoPortal without acknowledging it and stating its license would be a violation.
I recommend that whatever value added you plan to build, you should never modify mojoPortal code, all customizations should be in custom projects and plugged in without changing mojoPortal. Once you start down that path of modifying mojoPortal directly it is hard to get back on track and you will basically have a forked version with no ability to upgrade without losing your customizations.
Hope it helps,