IIT Mandi is a reputable institution with a strong focus on academics. Although it was ranked 43rd in the overall category in 2022, it's important to note that rankings can change from year to year, and they may not take into account all aspects of a university or college's performance. Additionally, opinions about the quality of an institution may vary depending on the individual and their specific experiences.
Honestly speaking, IIT Mandi is yet to establish itself and is still a baby IIT. Most of the good reviews are usually from the students of the institute itself and no outsider can confidently justify its credibility. And it is understandable since it’s a work in progress and credibility cannot be gained overnight.
The old IITs did not become what they are today just after 5-10 years of their establishment. It took them decades to make the mark and stand high among Engineering colleges. So it’s advisable you target older IITs or top NITs if possible and keep these new IITs as the last option.
Overall, it would be best if you could visit the institute and make your own assessment.