The short answer: yes, that's a little too soon- particularly, if you are expecting her to really and truly "sit up".
High Chair Manufacturer's Guidelines
Just take a peak at the high chair manufacturer's age recommendation. More than likely you'll notice they do not advise using a high chair before 4 months of age. Now some high chairs do have a recline position. Parents may find that feature convenient to use as a resting place for their baby. Perhaps the reclining high chair serves as a good seat with a view as mom or dad preps dinner, or works out well for those hurried moments when mom or dad is bottle feeding their baby with one hand while eating their own dinner with the other. However, I wouldn't consider that sitting up in a high chair. Further, you also should not use that recline position when you begin feeding baby food to your munchkin either.
How Well Can Baby Sit Up With Support
To know when your baby is ready to sit up in the upright position in a high chair really is fairly easy. Her physical development between 4 and 6 months should begin to reveal that she can sit up well with some support. She should show fairly good stability and control when seated, with only slight bobbing about. The ability to hold her head up is also a must.
A great tip for starting solid food is to get your baby familiar with being seated in the high chair in the weeks before you actually start solids. Let her take the chair out for a "test drive" and allow her to become comfortable in her new little throne. That will be one obstacle you've overcome when it comes time to start solid foods.
Related Reading: tommee tippee explora bibs: great bibs for solid food eaters.