A baptism is much more than a formal occasion!
And it does matter that it happens in church!
So it is very important that the OP wears what she is comfortable in, and that no one turns up their nose at her for wearing jeans!
Those who would insist that anyone has to dress up for church is missing the point.
James 2:1-4
Warning against Partiality
2 My brothers and sisters, do you with your acts of favouritism really believe in our glorious Lord Jesus Christ? 2 For if a person with gold rings and in fine clothes comes into your assembly, and if a poor person in dirty clothes also comes in, 3 and if you take notice of the one wearing the fine clothes and say, ‘Have a seat here, please’, while to the one who is poor you say, ‘Stand there’, or, ‘Sit at my feet’, 4 have you not made distinctions among yourselves, and become judges with evil thoughts?