I've been diagnosed with bipolar disorder. Is it bad to get into a relationship if I am bipolar?

Maybe, maybe not. It can be bad if treatment is either absent or ineffective to form a relationship or maintain one already established. When trying to form a relationship it would be wrong to not level about your disorder. Trying to maintain an established relationship is even harder. Choices are harder with more invested in an established relationship. A good relationship is only possible with frank honesty.
No. Relationships are associations between people that require a great deal of selfless participation. If your bipolar is controlled with medication & counseling, trying to maintain a relationship is no different than for any other couple. Be sure to let your companion understand that the mood swings are not caused by them. Choose carefully, but don't avoid a relationship. Discuss with your doctor.